Assuming its a stereo push-pull or PSE amp then yes, you have to change them in matched pairs (per channel) to keep distortion low and extend valve life, ideally you'd replace all four with a matched quad for channel matching. Doing this with used tubes is all but impossible without a good stock to sample from, and a tube tester. What is sold on fleabay as matched is almost always erm, umm, not quite exactly as described shall we say and EL34s tend to get a hard life so buying second hand is at best a risk. That leaves you looking at new which means NOS or modern production stuff. Modern stuff is getting better by and large and getting matched quads affordably is easy, the tubes WILL be new, and quite possibly well matched. NOS is often,erm, umm, not quite exactly as described shall we say unless you get them from a reputable trade source, and they will cost. So why bother? Well many think NOS sound better, and pretty much certainly the old Mullard/Philips/Siemens/ etc are better made with better materials and will last a lot longer, and have much higher resale values. Prices for old EL34s are further pushed up because they are used in guitar amps, and driven very hard in such amps for the crunch and breakup sound they can produce. Here again there are favourites, and NOS are favoured not least for recreating the authentic sound of sixties amps. Many here favour Svetlana winged C era Russian EL34s, and if you are lucky you can find these for sensible money. Some like Siemens EL34s (RFT made) as cheaper available NOS but this is subjective and others on here disagree. Pretty much consensus between hifi and guitar is that NOS Mullards sound fab, you can work out what this does to the price
