Hey Ronnie,
Just throwing something out there, I know you mentioned "limited hard". And that rings a bell from Bill F, and I see the DR260's you are using, and I know that Bill recommends pretty aggressive limiter settings, especially when mentioning horn subs.
One thing that you might be running into is you might be limiting the UCS1's so hard and when you start running into the limiter and keep pushing it the average "heating" power going to the subs is raised and you end up frying voice coils that way.
So I guess, what voltage are you limiting the UCS1's at?
Also what kind of material is this used for? Playback, live music etc? Also, what size crowds?
From what I see I would get use an amp capable for around 1000 watts to each UCS1, with *No* hard limiting. Just keep the amp out of hard clipping and you should be fine. Yorkville is pretty conservative with their ratings.
Honestly, a Pro9200 would be getting right about 1000 watts per driver with 4 subs on it. So maybe pick up a second Pro9200 for the other 4 once they are all fixed. The Pro9200 is known to perform well on sub duty as well.
Honestly going from a Pro9200 to a PL6.0 would be only a gain of ~1.5dB, not noticeable really at all.
So I would really just use the 2 Pro9200's for 8 UCS1, and take out any hard limiting on them and keep the amps out of hard clipping with proper ~35hz 24dB/octave HPF set, and you should be fine with most kinds of music.
Take Care,
Phil