What do you see as the downside of tubes?

I lost patience in trying to read all of the above - here are some random thoughts:

Viridian makes the most important comment in my mind - you have to spend a lot of money on a tube amp to get a good one, i.e., one having power supplies and output transformers of sufficient quality to enable it to properly drive most audiophile speakers. It's hard and risky to define "a lot", but let's say well north of $10k retail - many (if not most) tube amps are overrated and not understood by their purchasers.

Your Living Voice speakers, however, are excellent and present a benign impedence load, allowing you a lot more flexibility with respect to amp choice. I have no personal experience with the Mastersound amps. I do not want to steer you away from single-ended amps on Living Voice speakers, but a hassle-free tube amp that is really well built and sounds truly great is the VAC Renaissance 30/30 push-pull amp. It biases the output tubes automatically and has a tube shut-down feature that kills the power to any output tube that is about to blow (these features take all of the hassle out of owning a tube amp). All point-to-point wired (no boards on those babies). It runs four 300-B's and puts out 32 watts/channel - it would sound like a 600 watt/channel Krell on a highly efficient speaker like the Living Voice.

Despite the high initial downstroke, good 300-B's (Sophia Electric, Full Music, E.A.T.) will last a very long time (10,000+ hours / 5+ years).

While some tube gear is truly problematic to own, a high-quality tube amp running standard-fare pentode or triode output tubes is arguably more "reliable" in the long run than certain high-end solid-state designs - when a tube amp is retubed, it's basically a brand-new amp, while some high-end solid-state designs have been known to become irreparable because they use rarefied output transistors that go out of production. Also, you replace output tubes, while a $50/hour tech repairs the solid-state amp you had to pack up and send away to him.

A high-powered tube amp that uses a lot of output tubes can be a pain in the ass to own because of the higher likelihood of tubes losing bias and fact that easy biasing is not always a design priority (Brunhilde and Wotan owners with burnt fingers know what I'm talking about). However, if you are running a single-ended amp with one or two output tubes, or a high-quality amp that auto-biases like a VAC Renaissance or Audio Valve, there is little or no more demanded of the owner than with a typical solid-state design.

Regarding the comment by Markwatkiss about supposed fairy tales, high quality output transformers are mandatory (as are hefty power supplies) for controlling the woofers in the typical audiophile speaker featuring a 4 Ohm or less load in the bass, and they are extremely expensive. I am not aware of any tube amp that retails for less than $10k that can properly control the woofers of such speakers assuming a normal listening room and average program material.

Ralph (Atmasphere): I agree. I have owned auto-biasing VAC Renaissance 140 monoblocks (sixteen 300-B's) or a VAC 70/70 (eight 300-B's) for almost nine years - they have been completely trouble-free. Having an amp that, if I may quote you, "is designed properly" is the key - cheap tube amps do not play in your league.

Krauti:

You cannot run a tube amp 24/7 because the output tubes pass a lot of current and will wear out too quickly. This is generally not the case with gear like tube preamps or tubed DAC's that use small-signal tubes (little tubes) - 24/7 operation with small-signal tubes usually extends life and yields better sound, as turning gear on and off subjects the tubes to harmful thermal cycles, as well as a voltage rush on power up if the gear uses solid-state rectification, which is most gear. The latter factor is the biggest problem, which is another advantage of tube rectification and why a lot of tube preamps have a "soft-start" feature which applies voltage slowly to the tube filaments on power-up.

And as others have noted, a tube amp with a lot of output tubes will heat up a room (extreme cases, like ARC's 600 series reference amps, basically require a separate room or custom air conditioning unless you are running them in a gigantic room).

I have a solid-state amp (darTZeel) and tube amp (VAC Renaissance 70/70) in my living room, and run the solid-state amp most of the time because, like the poster who spawned your question, I like the convenience of being able to listen whenever I want. The VAC sounds good in 10 minutes, but sounds better after a few hours of operation, and I don't like powering up the amp and exposing it to a thermal cycle when I'm only going to listen for a few minutes. I do run the VAC in the spring and summer when there are frequent electrical storms, as I work far from home and am not able to get home to power down the darTZeel during electrical storms.

Just to be clear, the issue with tube amps and 24/7 operation is that the big output tubes pass a lot of current, even while idling, and this creates undue wear. Small signal-tubes, on the other hand, pass very little current in most circuits, and as I have written ad nauseum in other threads on this forum, when left on 24/7, they tend to either die within 200-250 hours from "infant mortality" (defective manufacture), or they basically last forever.