These are an interesting match for the Peavey 6505 amps. I have never really heard of Sheffield speakers other than Peavey using them for certain amps in their line. I don't know a whole lot about them but match with the Peavey 6505 amps they are definitely a good fit together. The sounds is loud and thumping and that is the best way I can describe it. They have a punch to them that is similar to a British sounding speaker but with a crisp high end that matches the voicing of the 6505 amps perfectly. I have also heard the 6505 amps with V30 from Celestion but I think these speaker give the necessary unity with the amp.
What I prefer these straight cab over the slant are two things: one being that it is $50 cheaper for the straight cabinet. The other being I feel that the cab retains some more of the low end thump for the amp much better than the slant cab does. The is typical for just about any manufacturer. You get more wood and the speakers are all more directional in one location rather than 2 going one way and 2 going another way.
Each speaker I believe is rated at 75 watts, but I'm not sure since I have never opened the back up off of these cabinets. The construction of the cabinets are pretty decent, but they certainly are no Bogner cab, a Mesa Boogie cabinet, or a Mills acoustics cab. They match the amp but I'm not sure how well they match other amps. My guess is that if you buy this cab it is going to be because you have a Peavey 6505 or 6505+ and you want to get what the manufacturer intended.
Peavey 6505 Straight Guitar Extension Cabinet Features at a Glance:
Straight, closed-back guitar cabinet
Made of high-density plywood construction
Loaded with 4 x 12" Sheffield 1200 speakers
Locking XLR or standard 1/4" inputs
300-watt (RMS) power handling
High-density plywood construction
Speakers 4 x 12" Sheffield
Cabinet Type Straight
Construction Material Plywood
Inputs 1 x XLR, 1 x 1/4"
Impedance 16 ohms
Weight 90 lbs.
At new these cab come in at around $699 which isn't bad for a 4x12 cabinet. Marshall cab come in well over that price. So you could probably pick one of these up on the used market for half that since Peavey doesn't retain their resell value all that well.
I would recommend this cab to a Peavey 6505+ or 6505 owner or someone who needs a relatively inexpensive 4x12 cab. I don't have much info on the speakers since the brand name is not familiar to me. They do sound pretty good though coupled with the appropriate amp.