During the weekend I spent some time comparing my two amps, using the 009s. The STAX SRM600ltd has just gone back from being serviced by highendworkshop, which is the official STAX service company in UK, for a problem to the right channel. The problem has been fixed completely and the amps now works fine.
The difference between the STAX and the Megahertz is larger than I remembered, but in the meantime I've rolled some tubes and completed the burn-in on the Italian.
The 600ltd is tonally brighter, with a more distant presentation. Timbrically they are similar, with the Megahertz richer and more liquid in the midrange. Instruments are also more 'stable' with the Megahertz, more focused with a bit more air between them (not a massive difference though).
The biggest difference is in terms of dynamics, the 600ltd seems to struggle during the louder passages, loosing coherence. It is something I've never noticed until I heard the Guerra. During these passages the music seems also to become brighter, almost as a tonal shift, and the midrange grainy. This never happens with the Guerra, you can always follow the lines of any instrument, they are always clearly defined, and tonally is consistent. In fact, I tend to listen to a higher volume than I used to, which tells me the mid is a bit forward on the Guerra (or the high slightly recessed?). But in any case the Guerra sounds rolled-off or misses extension and 'air'...the top end is very very clear and smooth.
The other area is the 'bass': instruments like organ or cello really come alive, they are more present in term of volume (they are louder) and in terms of extension and texture. Never heard Handel's Organ Concertos this lifelike and effortless, a goosebumps experience!
All in all, the improvement is there, as I expected given the price difference, but the STAX amp is still an excellent, and cheap, choice which allows to fully enjoy the 009s...I have it for more than 2 years and always been happy. The only reason I'm selling it is because I don't use it and i can't afford keeping it :-)