I have some experience with different types of rectifer tubes in my amps. The 5y3, 5v4, and 5ar4/gz34 has a 2 amp heater draw, the 5u4 and 5r4 use 3 amp heaters. The idea is that if your amp comes with a 2 amp heater tube and you wish to experiment then it would be best to sub a tube with similar heater rating, to avoid overloading the 5 volt windings in the main power tranny.l 5y3 is weakest in transmitted current, 5ar4 is most in 2 amp heater types, 5r4 is stronger than 5u4 in 3 amp heater tubes.
Regarding vintage old stock 5ar4/gz34, prices have doubled in the last year on ebay. A spendy mullard, siemens, telefunken, philips, or real amperex can set you back a ton of cash and while they have the reputation of lasting longer than you will on this planet, remember that that life is different to tubes placed in a combo amp. Frankly, I'd not put a real expensive vintage rectifier tube in a powerful combo amp and let it get shaken to death prematurely. I would, however, put an inexpensive vintage tube in a combo amp and not worry so much.
5v4's have an output fairly close to 5ar4 and are much less expensive. Have had great luck with Sylvania and GE. 5r4 has more oomph than a 5u4, and some, notable the Cetron military "potato masher" have a very massive glass bottle and base. If you have a tube clamp as part of your socket, they won't fit. Other 5r4 tubes have similar size and shape as 5u4. The great thing about vintage 5u4/5r4 is that they are relatively inexpensive and plentiful.
The Mesa/sovtek 5ar4 is a copy of the Mullard sawtooth 5ar4 and is a relatively decent tube, and not expensive.