I got away with a 5AR4 for a while, but changed out to a GZ37. Difference was pretty obvious - the 5AR4 was really hitting the stops, and you could tell the amp was breathing hard. The GZ37 can pass a lot more voltage and really makes it sing.
The WZ68 copper cap is another good option, and a lot cheaper. But ... it doesn't glow in the dark, eh. ;-}
With either "upgrade", you can set the bias to 60mV comfortably. That bit of a boost gives the KT120's a bunch more punch. Don't worry about over driving the tubes ... ain't never gonna happen in any KT88 based amp ...
And ya ... I crank it up to "11" on a regular basis. Almost a requirement with prog rock, eh. That's with some fairly inefficient (89db) speakers too. I also use expansion (dbx 3BX and BoomBox) which pushes it even harder, and nary a problem.
No TDR here, but I do have a CL90 thermistor on the power line. That gives you a slow(er) start than a direct hookup and is a real simple mod. The amp does have a higher rated amperage than the CL90, but I've never had any issues. You DO need to provide plenty clearance as they do get HOT in operation. You can get higher rated thermistors, but they have less surface area to bleed heat, so it's kind of a wash in my book.
NOTE > > If you get a copper cap solid state rectifier (and it's highly recommended at least as a spare), their site lists the WZ34 as a 5AR4 replacement. The WZ34 is fine in a smaller amp, but the big rigs can push it pretty hard. The WZ68 can handle a lot more power - the equivalent of TWO 5AR4's - for the same money ... kind of a no brainer really.