I picked up a "vintage" Yamaha YBA-100 bass rig a few weeks ago and am in the process of tricking it out. The head is a 2 channel preamp and the cabinet contains the power amp and 3 x 12" speakers. The power section is rated @ 100 watts RMS [I have the SM] but the speakers are 50 watt, 8 ohm wired in parallel - so this would mean it's running closer to 300 watts near as I can tell. This unit was sold in the early 70's as Yamaha's ultimate bass rig. It's survived the years very well. A buddy used to be a Yamaha tech during the era this was current, so we'll be doing a cap job and changing the trim pots to bring it up to spec. As far as the cabinet goes, it's a closed back affair with the 3 speakers and a rectangular hole at the top of the baffle. I'm not sure if this was meant as a heat vent for the amp, or a primitive port. I'm replacing the 3 x 12's with a JBL K140 and 2 Peavey Pro 10 Neo's so the baffle has to be replaced which brings me to my question. The cabinet is built extremely well [this was a premium unit at a premium price] out of plywood, but what has me confused is the baffle - it's made out of a very poor grade of MDF. Not even good stuff. Does anybody know why Yamaha would opt for a cheap particle board baffle? I can't see it being a weight thing, as MDF tends to be heavier than ply and it certainly isn't nearly as strong. I'm leaning toward a 3/4" pacific birch ply baffle, but thought I toss this out to see if there's something I'm not understanding [which is highly possible]. And a ? about speaker placement. The 3 12's were placed with 2 along the bottom and the third around the middle - the power amp section takes up most of the top 1/4 of the cab. The Peavey 10's are the same depth as the Yamaha 12's [not sure yet about the depth of the K140, but my guess is it's deeper - it's still in its cabinet]. If it turns out I can't put the JBL in the center because of space, would there be a problem with putting the 15" at the bottom of the baffle and the 2 x10's in the center? Thanks in advance. PS - they also used these really strange bolts to hold the baffle to the cab and the speakers to the baffle. They have bare, flat heads with a twisty "nail" affair at the head-end. The idea is they are "nailed" in from the front and the twisty part is supposed to hold the bolt in place. That's the theory, but in practice, due to the cheap particle board, they simply eat away and strip the wood when even a small amount of force is put to them. Anyway - I need to find something similar, or a replacement type of bolt that will let me attach the new baffle like the old.