It varies by phono stage and it's definitely something that not often specified (even for stages that have filtering), but it would be much more useful information than the usual "RIAA within +/- 0.5 dB" stat lines. I use a Rogue Ares, and because this was a concern of mine, I remember having a call with Mark O'Brien (about the time of its initial release 2010) where he said something along the lines of (paraphrasing here) "yes, it has a subsonic filter, and I don't get why some makers don't use one". I didn't ask for or get any specific numbers on its rolloff slope, though.
I still do see some woofer pumping with my current setup, especially on certain albums (yes even with the ring clamp) and the volume turned up high -- but rarely is it severe enough to cause concern. This is something where a little goes a long way -- you want some subsonic filtering to reduce the woofer pumping, but too much and you'll certainly start affecting the audible band in less desirable ways (as with anti-skate -- you want a little, but definitely not too much). Ideally a phono stage would provide adjustable filtering. For a fixed setting, I think the Ares strikes a decent balance, which is to have some moderate filtering. I've had phono stages over the years where the woofer pumping/flapping was certainly more noticeable -- Sonic Frontiers Phono 1 and original Hagerman Trumpet come to mind, though they were still very nice stages overall (just be quite careful not to play too loud on the "bad" records).
A quick lol at me: I've bought the Rogue Ares THREE times, but this time it's sticking around -- changing speakers 4 years ago made it hard to settle things down again, for a while. That alone should stave off speaker upgraditis for a good long time.
Subsonic filtering is the last resort and definitely an after the fact cure, not prevention of the problem. Although good and beneficial if there are no other means to combat the problem available, it is still a band aid - at best.
Every phono cartridge/tonearm has a fundamental system resonance, governed by the compliance pf the stylus suspension and combined effective mass. It should, ideally, be in the 8 to 12 Hz range, with 10 Hz being an ideal value.
What is NOT specified as "ideal" is the magnitude to which this resonance amplifies the output relative to the level in the upper bass/midrange and its sharpness or Q - the quality of resonance. Also, the sheer excessive excursion is inversely SQUARE proportional to this resonance frequency; meaning the excursion of your woofer gets severely more pronounced if the resonance of your cartridge/tonearm is say 8 Hz ( or lower...) than say 12 Hz.
To combat this undesired effect, there are several options available prior to going to the last resort - the subsonic filtering, which is nothing else than sweeping the dirt under the carpet.
1.) Silycone Oil damping ( a la SME, in recent(ish) times KAB for Technics SL-1200 , Mission 774, Moerch, etc, etc ). Better something than nothing. It can impair a VERY bad influence on sound overall by making it "dead" . It proved that excessive VERTICAL damping is to be blamed for this - and more refined versions use no or use adjustable vertical damping in order to bring the best results using this method ( Audio Technica, Moerch ). I remember measuring Moerch DP6 with a few cartridges - adjusting the vertical damping from zero to maximum had an effect of reducing the resonant peak by the "whole" of approx 1-2 dB ( almost negligible at peaks being usually + 10 to + 14 dB relative to midband or should be 0 dB value ), whereas the subjective effect was HUGE. From overzealously open to decidedly dead. An extreme case of silycone damping is Townsend - with a trough in front of the cartridge and plunger attached to the headshell. No direct experience with this one.
2.) "Something" attached in the vicinity of stylus - from the simple "brush" arrangement of Stanton/Pickering trough more sophisticated Shure version called Dynamic Stabilizer ( which is a carbon fibre brush vertically damped with sylicone ), trough universally applicable Discwasher Disktracker ( here used WITH Stanton Brush !!! .... ) : https://www.youtube.com/watch?v=hLy48M2wkFQ
and variation on the theme by Zerostat http://www.freepatentsonline.com/4211422.html - to the top of the hill air damped Stax CS-1 ( version specifcally for Stax CP-Y/CP-X II cartridge ) or universally applicable CS-2.
This method is more successful in reducing the amplitude of the unwanted cartridge/tonearm fundamental resonance than the sylicone damping. It has the unfortunate side effect of MANY ( Shure claims IIRC 10.000 bristles in its carbon fibre brush ) "styluses" tracking the grooves roughly 3-4 mm before the stylus proper can reach the groove - inevitably transmitting vibration to the cartridge body, thus picking, albeit at low level, an "average" what is going in the grooves tracked by the "brush". It IS audible, on music most noticeable when the stylus proper is still tracking a very quiet portion of some classical music and "brush" tracking say a loud finale - you WILL hear it. It is noticeable with less difference in level between the stylus and "brush" portions of the groove, too - but to a lesser degree and to a more attentive listener. I do have Disctracker (still NOS virgo intacta ) and therefore have no idea how loud its "brush talk" is; Shure*s is bad enough in V15IV ( where it was first introduced ) and V15V ( and lower models intoduced at the launch of these two V15s ), only to be made worse still with the last true generation of quality Shure cartridges - ML120He, ML140He, Ultra 200 and Ultra 400 ( they differ in stylus and cosmetics/lettering ) - where this "brush talk" is excessive. There is a reason Stax CS-2 commands such astronomic price http://www.ebay.com/itm/STAX-CS-2-Air-Damped-Cartridge-Stabilizer-/261934403157 ; it was the rarest of them all, the most expensive at the time of introduction, I have heard rumors that German distributor sold an entire quantity of 6 ( in a word : six ! ) samples throughout the history - but it works and has the lowest "brush talk" of any of the devices mentioned.
3.) Dynamic Damping/antiresonator approach. This involves (usually tunable ) arrangement spring/damping controlled resonance of the counterweight of the tonearm. It was a feature of a very few TOTL tonearms ( Technics EPA 100, EPA 100 MK2, EPA 250, Pioneer ???, Denon ???, EPA 500 series, few Dual top/upper models with Q suffix ) which feature this damping in both horizontal and vertical direction. It works by adjusting the resonance of the counterweight EXACTLY at the resonant frequency of the cartridge/tonearm - both in frequency and quality - in theory, both would cancel each other EXACTLY out, yielding a totally flat response (and an absolute minimum of woofer flapping ). In practice, I achieved results of peak at resonance being approx +3 to +4 dB - which is a HUGE reduction of amplitude of the resonance compared to the usual +10 to m+14 dB; if you recalculate these dBs into percentage, +4dB is approx 20 % error from should be value and + 10 to 14 dB is, roughly, 400 % (!!!) error !!! This is the best resonance control option ever made available in a commercially available product. A bit less sophisticated ( only the vertical dynamic damping - which is the most effective/beneficial ) was available in the only relatively inexpensive turntable - the Tesla NC-470/NAD 5120/Lenco L-802 . This damping can be adjusted with the extreme of precision - yielding, for all practical purposes, almost totally flat response in the vertical mode - no resonance measurable, with any cartridge.
The drawback is that this adjustment absolutely has to be made with utmost precision - or you can end with TWO instead of one resonance, making things far worse than with a tonearm with "stupid fixed counterweight". A test record is mandatory - and, even if you are experienced, it means trial and error of adjustments, until you grasp how adjusting the resonant frequency AND Q interplay/effect the total response. A NAD 5120 adjustment ( after geometry, VTF, antiskate, etc has already been dealt with ) of cartridge/tonearm resonance - or better said its cancellation - can take up to 4 hours ( ! ) - but, make no mistake, records will then sound quieter as far rumble and woofer flapping is concerned than on say 100.000$ ( or more...) turntable fitted with a tonearm that takes no measure to combat cartridge/tonearm resonance. A properly adjusted NAD-5120 ( the name under which the original Tesla NC-470 is generally known in the West ) is among the best turntables available - despite its rather chintzy build.
4.) Electronic compensation for the cartridge/tonearm resonance. Sony Biotracer, Denon, JVC. It works similarly as above, much easier to adjust, but has the tendency having not enough "brain" to discern which vibration it SHOULD suppress and which it should let them be. It can manifest itself in reduced perceived dynamic range.
No direct experience with these systems - only the comments I could find online.
5.) Eddy Current Damping. The sole commercially available product is the Dynavector family of tonearms - starting with the original DV-505. These arms use eddy current damping in horizontal direction - and are perhaps the only commercially available option to play back the most demanding record of them all - https://www.discogs.com/Lloyd-Holzgraf-The-Power-And-The-Glory-Volume-1/release/4537692
It has recorded bass down to ( forgot the exact Hz - but BELOW 10 Hz ) and will unsettle anything else - short of possible something with electronic compensation as described under 4.)
Remember, in order to fully understand and appreciate what was written above, one must absolutely understand
http://www.theanalogdept.com/images/spp6_pics/TT_Design/MechanicalResonances.pdf
It IS technical, it IS NOT an easy read - but it is the Holy Grail of analogue disc playback. Do yourself a favour and "digest" it - it can provide answers to your concerns and can save you very costly mistakes.
It is not a great feeling when your 100.000+$ TT gets clobbered in a direct shoot-out by a ( PROPERLY adjusted ) "little humble" NAD-5120 ...
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) and one is an AP 45 RPM Art Blakey (moanin). No audio problems I can hear though; just looks irritating. 
