Those Sheffield cabs do sound quite dark. I like them though. Not sure why people rag on them. I think it just became a trend to criticize Peavey stuff. Probably a campaign started by Marshall ;)
Congrats! If you like them, there must be a lot of Peavey-fans all over the world. Speakers are all about preference, which music you play, what guitar you're using.
I never changed the speakers in my Peavey Classic 4x10 I had a few years. They sounded good to me. I just bought a Fender Champion 110 for my nephew's birthday. I must say the stockspeaker sounds nice and full. I was surprised by the clean tones of this solid state amp. Paid 50 euro for it.