I would change all five - including the output driver which IIRC is the one on the far right hand side looking at it from the back.
Mel has a good point about checking the emitter resistors (the .27 ohm 10 watters that bridge between the board and the transistor sockets). Measure the value once the transistor is out of the socket. Generally, if these are bad you will get an infinity reading (open - as opposed to a short).
Do not even give a thought to matching transistors.
There
will not be any increase in output power by substituting output transistors.
Nothing wrong with using stock replacements (i.e. 2N3055). If you can find 2N3773 types (16A 140V 150Watts) don't hesistate to use them as they will be more durable... but you are really just wasting money using higher ratings/higher cost transistors like MJ15024 types.
No suggestion as to brand. I haven't found any reason to think it makes any difference. I order from Mouser... and I usually just order the cheapest transistor of the type they have in stock. (
Maybe in the future we will be arguing over JJ and Ruby and NOS style 2N3055s 
but not today)
While you're at it, if the amp has been sitting awhile you might want to give some serious thought about replacing the power amp electrolytic (usually 3400 @ 75 volts). This will probably cost you more than all of the transistors combined, but I would do it anyway.
Tips:
Use new micas and thermal compound - make sure to clean the old stuff off well. Don't glob on the compound as all you need is a thin film. Clean off the excess that will squeeze out once you snug the transistor down.
Don't mix up the emitter/base pins and try to force it into the socket. Don't overtighten the screws, especially the first one but make sure the first one is snug before putting the other in (because of the cover)...
and
do not leave off the cover in hopes that the transistor will stay cooler.
