Mrtennis, Consonance tube amplifiers in general are extremely good products, from the standpoint of both build and sound quality. Unlike their CD players and solid state or hybrid amplification, I can think of no particular reason not to consider these tube amplifiers, as their track record has been enough to make me feel comfortable.
The sole area of concern I have for the generation of tube amplifiers you are considering is that the bias resistor values seem to be all over the place. By that, I mean, the schematic specifies a certain value, but I've seen three different values present in the various amplifiers that have crossed my path. It seemed as if they were trying to figure it out on the fly. And, 2 of the 3 values, including the schematic, are not the optimal setting, one of them clearly being more than a little bit wrong. What are the downsides of that? Well, in the worst instance, there was a more than noticeable hum. And, in the most minor, the amplifier just ran hotter than it should. Of course, I didn't have the product long enough to know if tube life would be compromised, but we did make sure to retrofit the amp so that it was now correct.
The M400 monoblocks, while quite nice, have been eclipsed sonically by some of their more recent designs. By that, I mean, the refinement, detail, and clarity have been improved, in that tired old "lifting the veils" audiophile cliche. I'll put it this way, that misty/hashy quality is reduced to great extent. However, not at the expense of musicality.
In fact, I find the Cyber 800, which my friend Mariusz (Mrjstark) owns, are without a doubt the best push-pull tube monoblocks they've produced. They have a nice mix of richness/musicality with the clarity and detail that modern amplification must possess. I'll leave power out of the equation, as I'm not sure it is or is not important to you, but they are plenty powerful. One thing I want to throw out there is that the Cyber 800 ARE NOT compatible with 6500/KT88/KT90 (or, JJ's version of the KT77 for that matter), despite some the bad information put out there.
On a more affordable plane, though the Chinese high-end audio companies have completely lost that advantage over the past 2 - 3 years, the 15th Anniversary products (integrated and stereoblock) are the first which I feel offer the realization of putting it all together in a package that doesn't require the commitment of both size and money that their Cyber monoblocks demand. They combine pure Class A operation, large power supply, and superior transformers (supposedly, amorphous, but I've yet to verify it 1000%) which make the Cyber 800 special, with superior coupling caps (AuriCaps, instead of the Cyber 800's Solens), tube rectification (normally, to me a tradeoff of bass for that beautiful midrange, but no tradeoff in this instance), and the owner's (who is the designer) improvements circuitwise (he's really been studying up on the classic American hifi components over the past year or two). On several levels, they are the best tube products Consonance has ever put out.
Regarding the Quicksilver amplifiers, I am certainly a fan. When you buy a product, you're buying a company (that should be a caveat for those considering Consonance), and Quicksilver has certainly earned their sterling reputation. Quicksilver amplifiers are quite nice to begin with, but some owners like to tweak them a bit to really bring them forward. After all, Mike Sanders is building them to an incredible price, which he should be given a tip of the hat for. But, they have a bit more inside of them waiting to come out. The overriding thing is that Quicksilver products certainly earn my recommendation without any hesitation.
But, to answer your question specifically and succinctly, if it were me, I'd probably choose the Quicksilver mid monos over the Consonance M400 monos.
DISCLAIMER: I am the importer/distributor for all Opera Audio/Consonance components in the USA and Canada. And, of course, all of this, beyond the Consonance repair history, which I have statistical information on, is my opinion.
