I use the MB-185 Signatures to drive my Thiel 2.2's with excellent results. The 2.3's are somewhat harder to drive (from John Atkinson's Stereophile measurements), but I think they would probably be well within the 185's capabilities. VTL recommends this model for speakers with woofers up to 8", which fits the bill here. Although I haven't compared the ST-150 with my speakers, it would probably work fine as well, but the mono's will give you more power supply capacity and supposedly better output transformers. Neither have I auditioned the 450's with the same speakers (they were above my budget anyway), which use only one additional pair of output tubes (8 vs. 6 total per side) to develop their much higher rated power, but also feature much larger power and output transformers and filter cap's.
I suspect that the 450's and the 185's could sound noticably different from one another, but wonder whether with certain speakers or rooms where the extra watts aren't needed the edge would always automatically go to the more powerful amp (I almost said bigger, but they're exactly the same size, save for weight). Judging by the tube complement and the manufacturer's specs, it seems as if the 185 must employ a more conservatively-rated implementation of the same basic circuit design. The 450's are rated for roughly twice the power with only 33% more output tubes, which is basically in line with their ratings of their Reference double-decker amps, the 750's having about 3 1/2 times the rated output power of the 185's using only twice the output tube complement (12 per side). I don't really believe any of these nominal power ratings from VTL, which are consistently higher than you will see from other manufacturers using similar tube complements, although of course that's got nothing to do with sound quality (curiously, the 185's are rated at 220wpc in tetrode, but the 450's are rated at 450wpc and the 750's at 750wpc, so don't ask me what their numbering system derives from).
(FWIW, when Michael Fremer wrote a few years back about the then-new 450's compared to the 185's immediate predecessors, the 175's, he said he found the 175's to more effectively get out of the music's way or words to that effect, and those amps lacked the Signature output transformer revision. On the other hand, Chip Stern seemed to come to the opposite conclusion more recently. As usual, try to listen for yourself using your model of speakers if you possibly can.)
As for the Svetlana 6550c, industry consensus seems to be that this is the best production version of this tube type, and when I retubed my 185's with Kevin Deal at Upscale Audio, he didn't prefer - and recommended against - any kind of substitution here, including the new EI KT-90's.
