To me the main factors are sound, and I have not checked the transformer heat. But the sound indicates a progression from 5U4Gs doing the most (presumable current-wise too), 5AR4s in the middle, and 5Y3GTs the least, . They all work well in the Torii depending on what you want from them and the synergy with the rest….something I can't over-emphasize from my tube play. A not great sounding tube in one arrangement, can sound amazing in another. This is especially real with the Torii since it very transparent and revealing, has five tube sets, and some very useful switches and knobs to tune things with.

Like with VRs, the biggest general sound differences as you tone down the rectifiers are lighter/tighter bass, and an associated mild reduction in body and dynamics, and an opening/brightening in the midrange and highs. Pretty subtle and still totally dependent on the particular tube within the type, but these are generalizations I have noticed. I like things about all the types, but end up using 5U4Gs and 5AR4s most of the time.  

For me, the Bendix 5Y3GT sounds awesome, with great warmth, texture and detail, but I end up craving a bit more punch and power. 5AR4s seem perhaps faster than 5U4Gs in general and a bit tighter overall including a bit less deep bass. Perhaps as big a tendency is in the midrange, the 5U4G tending toward liquidity and the 5AR4 tending more toward detail and maybe texture. But this is hair splitting stuff and only my observation based on limited experimentation. I have two 5Y3GTs, three 5AR4s, and three 5U4Gs (two of which being I think 40s and 50s variations of RCA and all ST/coke bottle.)

The thing that can really be cool is using these different rectifiers to taylor your amp's signature, but also, the stock Rubys sound really good and Steve has arrived at a synergy with the stock stuff from his long experience and good ears. So I think this is a good start to get to know the amp. In fact, due to some other forum explorations, I reintroduced myself to the Ruby 5U4Gs, and thought they sounded really intensely in my face with the tubes I had in, and those amp settings. But now, after tuning the tubes/amp to them, I am enjoying them very much, and wondering what I found I did not like about them when I gave over to something else a long time ago. But I have more choices now with a lot of tubes to work with to get that magical synergy.

So it makes sense to me to get Steve's sound established in mind before playing around too much unless your tastes/system/ room won't let you do that.

Then, or after learning the designer's sense of the amp, the sky is the limit with tube play for this amp.

My experience is similar to will's. You will want the 5AR4 if you need the grunt, but . . . you may not need it. These are pretty darned powerful amps. If your speaker doesn't need more, and the stock rectifier has the right tonal balance for you, you can play within that tube type for a while. . . .

Hope your wait for the Torii is short!

Anyone have a pair of 5AR4s they'd like to sell?