The Polk Audio site has an active forum with many individuals who could be very helpful for this specific problem. Some I believe are retired Polk factory service technicians.
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I owned LSi25s for + years and probably replaced or had repaired, between the 2 speakers, the amps 5 different times.
I always dealt with Polk directly, but I think they are just a crappy sub amp. It's too bad because I really liked the look and sound of the LSi25s overall.
Average repair costs was usually around $100
Yeah the sound is great. I managed to fix the amp on my own.
I just took the back plate off and i think the board just has very weak heat dispensers. I pulled two transistors apart a little that looked like they had been moved closer together by heat induction, and bam. Fixed.
I actually just purchased 2 Polk LSIM707s, 2 LSIM703s, an LSIM706C, and a Aventage 3070 from Adorama for $2800. The deal is still going. Got lucky and they all arrived in perfect cosmetic shape.
Oddly enough though I still prefer the sound of the LSI25s to the LSIM707s. Maybe the need to just break in some more.