[quote name='umph' post='454728' date='Apr 5 2009, 03:36 PM']matched sets have a similar current draw, thus bias the same since most amps don't have seperate bias pots for each valve. getting unmatched valves can lead to some very nasty crossover distortion and the amp not putting out as much wattage as it could be doing[/quote]
Crossover distortion is a result of 'cold biasing'.
Most multi-valve amps don't have separate bias pots for each valve because you don't bias a single valve in isolation. And many older amps don't even have bias pots - they're fixed bias and you just swap out the relevant resistor. And on later Silverface Fenders, there's just a hum-balance pot, which fulfils the function of dialling out hum by [i]balancing[/i] the tubes. The whole point of balancing is that it recognises that tubes can be unmatched. Hence, you can pay through the nose for a 'matched set' or you can balance an unmatched set. And any good tech is going to check and adjust the balance, irrespective of whether the mfr says the tubes are 'matched'.
Agreed, in something monstrous like an SVT or a VBA, matching is more important - but for the OP's amp, it would only be ludicrously unmatched valves way beyond manufacturers tolerances for the type (or fitting wildly different types) which could be a problem, but, unless you're buying the cheapest of chinese firecrackers, the variation between two / four / six tubes on a single production run is unlikely to affect matters to any significant degree assuming the amp is correctly biased.
Biasing is far more important to tone and valve life than spuriously "matched" tube sets. Until Aspen Pittman came up with the 'matched tube' voodoo 20 or so years ago, output valves were pulled off the shelf and fitted any old how, then balanced in the amp. Stock Marshall Plexis - unmatched. Stock Tweed Bassmen - unmatched. Stock Top-boost AC30's - unmatched.
In fact, slightly unbalanced tubes can usefully emphasise even order distortion, which for many, including me, enhances the musicality of the amp.