VTL, MIT caps and how close is the 450?

I own the 185's with the standard caps (and agree with your assessment of the amps' sonic quality). From what I understand, talking to Luke Manley and some other Audiogon members, the improvements can be worth the upgrade. VTL actually offers a choice between a couple of different cap brands for the upgrade, each having its own characteristics, but both apparently being an improvement on the original caps (we're talking about the coupling caps only here). I've also talked about this kind of upgrade with local techs, and at this point don't recall exactly which caps are the ones VTL is dealing with - I seem to remember hearing the names MIT, Musicaps, Multicaps, and Auricaps (one of the latter may actually be the same thing as the MIT's). I may have this upgrade done in the month of June, and so could have more to report later...

Multicaps RTX for coupling capacitors are always one of the best options. MB185 to MB450 is only the matter with output power and so is with room you're going to have the system. In vast majority of cases MB185 is plenty-enough for even large size room and toughest speakers.
I also noticed that in VTL they use one coupling cap for EACH power tube. I opted having only one expencive for each phase when made mods on my VTL MB100. I.e. it's sufficient enough to have two coupling caps in general. They made a pre-cautions that if one coupling cap goes bad(certainly within the time) than only one tube and possibly power-dropping resistors will. In my amp(4 power tubz) I realy risk with one extra tube if coupling capacitor goes bad but in reality distortions are reduced by default(by removing extra caps)

That's an interesting proposition, Marakanetz, about reducing the number of these caps in each of my monoblocks from 6 to 2. I'll have to bounce this idea off the tech I'm considering hiring. BTW, I suspect that the sonic difference between the 185 and the 450 is more than just one of sheer power - after all, the 450 only has one extra pair of output tubes, but uses a much beefier power supply and output transformer, which probably account for why it's nearly twice the price (VTL recommends the 185 for speakers with woofers up to 8", vs. I believe 10" or 12" for the 450). While I think the 185's are wonderful and all I need for my medium-sized room and speakers, if I had large speakers in a large room and could afford it, I would go for the 450's or 750's (keeping in mind that the quoted power figures for all the VTL amps are likely over-optimistic in reality).