BSI Model 4 Hypex DS 8 Amp.Issue

Does anybody knows anything about the BSI model 4 speakers? I have a pair, since 2012, that I bought from a dealer which went out of business...., but the speakers were manufactured by PBN, a well known speaker manufacturer. The problem I have is that the two subs from the left speaker are not working since the amp is on and everything seems to be normal. On the Hypex DS 8,  the information available is very little. No schematics availability and or any info. regarding this amplifier. Does there is something I could do to check on this amp, before buying a new one? I was trying on contacting PBN unsuccessfully, they may help. I don't know if this amp has some kind of internal fuses that allows the signal from amp itself to go through. Any help will be appreciated.