Gary paid me a visit and brought his amp over. Mainly to hear how it sounded on my system compared to the AVA FetValve 550ex. So, we listened to the RM40's hooked up to all of my gear, w/the Siemen's tubes in the preamp and the amp. My listening preferences are for a more laid back sound than Gary, so I adjusted the pots on the 40's up a bit for the mid's and highs. After doing some pretty focused listening, we switched in the Spectron. It had a more immediate, more detail oriented type sound, with better, stronger transient ability and bass than the 550ex. It was also very clean sounding, with only the slightest hint of grain to female vocals. Unfortunately, it was also fairly sibilant in my system, enough that I would have to change my cabling and probably my preamp to be able to live with it long term. But, I really liked the other things that it was doing (very well). The leading edges of notes and sounds were somewhat emphasized, but that lead to the very immediate sound that I was really digging.
On a lark, I thought of putting the Electroharmonix tubes back in to the fetvalve. It was a bad idea to take them out, the 550ex sounded a lot better w/the EH tubes, a lot of the snap and drive was back in the music. It still did not match the Spectron in these areas, IMO (the spectron is truly superlative in it's grip and control on the speakers), but it was quite a bit closer than at first. On the plus side for the 550ex, it was smoother, with a richer midrange, I found it more "listenable" over time. I don't think the 550ex really gave up much in transparency and detail, but the details were more in the background, they weren't pushed forward as much as the Spectron did.
In the end, I liked both amps quite a bit. The 550ex fits my personal tastes a bit more, with a smoother, slightly more laid back overall sound. But the Spectron is a great amp and it's just going to be a matter of taste which one a particular person would prefer. Maybe Gary will stop by and give his impressions of the 40's and our amp switching.