Are Quicksilver power tube amps in the same league as VAC, VTL, BAT, ARC, ...?

Absolutely, yes! My V4's were marvelous sounding. Traded for Levinson after combining dedicated audio and home theater into one room. Regret getting rid of them every time I listen to music. 

YES!

Absolutely!

@incho - I don’t have much budget (under 5K) and my choice is either new QS or those other brands used.

Used or reconditioned units from "other brands" vs. Quicksilver new is how many end up buying QS. At first they are looked at as a "good value", and when paired with well matched speakers, and when the amps and tubes are well broken in, realization sets in that "hey, these amps sound good!". With great input/driver tubes, another step up.

With good tubes and my own cap upgrades (at your own risk), my Mono 120s and a good buddies Mid-Mono amps now sound well above their price point. Having owned several prior tube and ss amps, I’d now put my own QS Mono 120s up against other amps at 2-3x the price. Many of the QS amps in the lineup are nice, pairing with the right speakers for each level amp helps maximize the performance and sound. Best of luck.

Yes, Quicksilver amplifiers are that good.  I'm currently listening with Quicksilver V4, and over the years have owned Quicksilver mini-mono, Quicksilver mid-mono, VTL ST150, VTL MB100, and Manley Labs 75+75.