The Stereomour II with headphones?
For headphones with an AC filament supply (on the 2A3's) you need some very quiet 2a3's with most headphones.
My SII was build for the AKG K1000's headphones and for now it's performing very good with these (no DC filament needed).
It is absolutely 100% silent with them, no hum, no nothing (104db/w/m). So the DC supply will be left out for now (same with AKG K340's).
The EML 2A3 solid/mesh are supposed to be very quiet tubes, these can be tried with more sensitive headphones in an AC filament SII (starts hammering the piggy bank).
The K1K's are very very transparant headphones with a gorgeous soundstage!
Some find them lacking in bass, but even with the driver angle's as wide as possible, i'm enjoying them with the SII.
The sound is rich, warm, full, and yet unbelievably detailed. This combination bests even the HD800/Mainline, loving it.
How about upgrades?
The SII has the biggest advantage over Stereomour I, in that it was designed to incorporate new BH upgrades and making the essential caps more accessible.
The first and foremost Shunt Regulator upgrade is a big upgrade with this amp!! C2A calling..
The DC filament supply and Mourquiet (a shunt / series stepped attenuator) are the other official upgrades (and i have no plan to install these for now).
Capacitors to upgrade: interstage coupling cap, outputstage/parafeed coupling cap, cathode bypass cap (2A3) and the PS caps.
Resistors to upgrade: attenuator (Mourquiet), cathode resistor (driver stage, output stage) and CCS current setting (R1) resistor.
The most important capacitor is the interstage coupling cap and i soon replaced the stock with K40y-9 Ruskie's.
The soundstage sized up, details came out but the tonal balance is not my preference. There's a loosening of the bass, with an unnatural tone to piano/strings from time to time.
I'll leave these in for now to give them time to set-in a bit and change them with Mundorf Silver Oil somewhere in the next coming weeks.
In the PS the last 100uf cap before the 2a3 parafeed choke was replaced with 470uf because i had the room for it, with little effect (beware the DC filament supply won't fit any longer).
For the output stage some Jupiter parafeed caps are planned. Can't wait to install these.
For the bypass cap on the 2a3 cathode (conveniently located at the front of the amp), four leftover MBGO's from the Crack will be used (if i can get them to fit, another reason Mourquiet wasn't installed). This can only be done after my coupling cap journey is complete.
Tube rolling is a very worthwhile activity with this amp. Driver tubes are readily available and cheap to try with a good influence on the sonic character of the amp.
The supplied Sylvania's were changed with Philips ECC81 (1957 TK1's D foil getter), Telefunken ECC81's and GEC A2900's.
All three lived up to the expectation to be an upgrade from stock. The GEC's and Philipses were magical with both headphones and speakers.
The Tele's (unexpected) were less preferred with the speakers / room because voices were placed so far back, they sounded muddled.
I can definitely see the SII as an astonishing amp for most headphones (DC filament required).
On the Bottlehead forum you can read all about
@Deke609 's adventures with the SII and Audeze LCD 4's.