They are quite different. The LPS' aew balanced and a bit cool: Telefunken-ish. The T-S have more bass weight and the most gorgeous Amperex-ish mid-range.

Out of curiosity, were the Sovteks indeed the 12AX7LPS? Typically the LPS sounds as smooth or smoother than JJ 12AX7s and is very quiet unless you put 'em in a high powered combo amp and crank it up, because those looooong plates will resonate.

OTOH, the Sovtek 12AX7 WA and WB are just as you described, grainy, harsh, and noisy. JJ 'AX7's will slaughter 'em. If I'm not mistaken, one of those is the stock preamp tube in the 5150 nowadays, or else they used to be.

Thermionic

Sylvania STR386 6L6GCs would sound fantastic in that amp, if you could lay your hands on some. I know, I've heard them in a 5150 before. It was the original tube the amp came out with, until Peavey's stock dwindled. Peavey had commissioned an entire production run of the 6L6s from Sylvania way back when, but by the time the 5150 came out it was the only Peavey amp they were installing them in. You even had to show proof of 5150 ownership to get replacement Sylvanias from Peavey.

That was a marvelous sounding tube. IMO, they were the best sounding 6L6GC I've ever used in guitar amps, regardless of the amplifier. I wish I knew then what I know now, because I pitched many sets that just had the new worn off them and replaced them with brand new ones. When I was a youngster, you could buy them for only $15/pr, so once they lost a little tone you could pitch them. Now, they're nonexistent.

A friend of mine who owns a Peavey dealership has tons of them stashed back, but won't part with a single one. He has done both ear and electrical testing on well used ones versus new production 6L6GCs. The worn Sylvanias trump the brand new current production! How sad you can't get tubes like that any more. The closest thing I've heard to them is the winged C SED 6L6GC (which was patterend after the Sylvania), but it's just not the same.

Thermionic