I've been modding my cab recently, and I'm thinking about changing out some of the celestion g12t 75's I have in my marshall 1960A. I was thinking of going for some kind of X pattern and keeping two of the originals speakers in, is that a good idea? and what would you say are some good speakers to choose? I quite like the sound of the vintage 30's, nice bass and mids, although I thought they sounded slightly harsh in the treble.

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oh they do. i've been standing infront of them all day, they definately sound better from the back or sides. Apparently the warehouse v30 clones have fixed that problem.

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Just rec'd my pair of WGS V30 clones on Thursday (Veteran 30's), and X'ed them in my 1960B cab w/ the existing 75's. Definitely smoothed out the mids, and from what I could tell, no harsh highs. I still need to really work them in, but so far so good. Very reasonably priced, shipped quick and well packed. I imagine these or those Hellatone 60's may interest you.
Good Luck!