Advice needed for replacing old ER3055 transistor
The 2N3055 is quite a common output transistor for audio gear (I actually have a bunch of boards sitting right here for a preamp I am designing that use 2N3055's)  

I think it was RCA who orignally made them, but now they are made by ST, On Semi, etc. From what I have recall the specs can vary very slightly between manufacturers (although I just checked the ST and ON Semi datasheets quickly and couldn't see any difference at all between them) but it's extremely unlikely to be enough to affect the performance of your amp (unless it's really being pushed to the limit)
So the answer is yes, any 2N3055 should work fine as a replacement in your amp. Note that the collector is actually the case itself in the TO-3 package, so make sure you bolt it firmly to whatever it's mounted on to get a good electrical contact (if the legs are soldered to a PCB, bolt first, then solder the legs to avoid putting strain on the solder when you fit the bolts if you were to do it the other way around) Also take careful note if the TO3's and heatsink are ISOLATED or not with mica, and/or dielectric grease, and/or the screws are PROTECTED by a insulating sleeve. Failure to note these details can destroy the amp further during re-assembly by shorting the output power circuit to its negative partner, ground or worse! (Dynaco brand units in particular)