I agree with MrPolatis. I have a pair of STR-425's that I've only used a couple of times. It is the quickest way to take the Mesa sound out of your Mesa and make it sound like a hotrodded Marshall is the only way I can describe it. The STR-425's (Sovtek 5881's) make for the perfect "brown sound" imho, as for the clean tones, as he pointed out, it rolls the highs off a touch and is geared a little more towards jazz/fusion, but honestly on the Mark series, being very pointedly clean anyway, I couldn't tell a heap of difference outside the R2 and Lead channels. They are a pure rock and roll tube. No heavy metal there.
The STR-440's however are more well rounded (being modeled after the Sylvania 6L6GC, but being Chinese made). While not having USA longevity, they will give you the rich creamy tones with a touch of nice modern compression. The bass is reasonably tight with lots of mids and smooth highs. Only complaint is that my pair sounded a little fuzzy, perhaps biased too cold. Still not Sylvanias (what I'm running) but does not carry the pricetag.
