Here's my personal notes on the MK IV SE vs Solo SRG. My SRG had about 250 hours of burn-in when I made these observations. I used my K702 with the Black Dragon cable and my RS1 on both the amps.
The SRG has a deeper but tighter bass i.e you will hear more "thump" from the bass drum but less bass guitar boom - perfect match for the RS1. The bass from the RS1/SRG combo is heavenly to my ears.
The SE has a noticeable hump in the upper bass/lower mids which gives you the impression that bass is not as controlled as the SRG. If you enjoy the bass guitar in rock music, you may well prefer the SE in this area. Especially if you have a K701/702
The SRG has a natural warmth in its presentation of vocals. Vocals are nice and full, but you don't get the impression that the amp is adding to the mids. It's more forward than the SE i.e. the singer feels closer to you.
The mids of the SE are very seductive. Smooth and sweet. Noticeably warmer and more full-bodied. You know it's the amp's euphony at work - but it's very enjoyable. Strangely, I find the SE a tad more laid back than the SRG (the singer seems further away than the rest of the instruments), but I've been advised that tube-rolling will easily cure that.
Ah, highs. The SRG has the best treble presentation of the amps I've heard to-date. Nice sparkle and decay in cymbals without being overly bright or fatiguing. You will immediately notice this the minute you play rock/metal music through the SRG.
The SE's treble presentation is a little more laid back. Decay from cymbals is ever so slightly less than the SRG. It's very enjoyable in it's own right, especially for those sensitive to treble, but a back-to-back comparison will show the SE to have less sparkle and snap than the SRG.
No contest. The SRG wins hands-down. Having been nicely burned-in by now, the SRG even surpasses my CanAmp. You get depth, width, height AND air around the instruments. Even with Grados. Simply astonishing. Be careful with the K701/702 though - on certain tracks, I actually felt disorientated as it didn't sound natural.
SRG wins hands-down again. Can't exactly define it. It has toe-tapping / head-bobbing musicality that has to be heard to be believed. The SE just feels a little sluggish in comparison, especially on bass-heavy, dynamic music like metal.
I hope this doesn't come across as one-sided in favour of the SRG. I actually enjoy both amps because alot depends on what phones you pair the amps with. And the music you listen to. Besides, you can easily modify the sound of your SE by tube-rolling.
You do know what my conclusion is don't you?
You need both...
FWIW, most of the time my K702 is plugged into the SE while my RS1/MS-Pro is paired with the SRG. The SRG works exceedingly well with the RS1/MS-Pro, while the SE complements my K702 nicely.
I'm happy with my current set-up... for now....