MC275 KT88 tubes blowing left and right

I got 2 MC275 demo model (MKV) with all new tubes. Using them in Mono.

After running for 2 weeks without any problems, suddenly one of them is malfunctioning - 2 KT88 tubes got blown within 24 hours. The first one blown out is the 3rd from the left and the the 2nd one blown out is 4th from the left.

I got replacement from the dealer (my MC275 dealer is just 2 miles away form my home). But wondering why is this happening.

Any pointers (apart from faulty tubes) ? Is it because of -
1) The problem in jumper cable (may be short circuit) I'm using for connecting the speaker posts to run the amp in mono parallel ?
2) Amp problem
3) Problem in power supply ? I'm using a stripper from the wall socket. In the stripper I'm connecting power cord of both the MC275s
4) Anything else ?

Interestingly, after the 2nd tube got blown out (it is not glowing), the amplifier is still working and singing ? How could that be possible ?

Regards,
Sourav