Feedback blew my phono stage?

Ok....I don't know what's going on here but here's my set-up
HALCRO
On the right hand side is my 'nude' Victor TT-101 DD turntable supported on spikes surrounded by 3 three tonearms on separate solid bronze armpods.
The 3 arms are connected separately to the Halcro DM10 preamp's phonostage via 2.5 metre long balanced Cardas Golden Ref phono cables.
With MM cartridges (I have around 30).....there are no problems.
With LOMC cartridges however......if I rest the stylus on the record without the motor turned on......I can induce a low frequency feedback through the speakers if I turn up the volume sufficiently?
Nothing I did to support the turntable differently.....could eliminate this problem so I have simply been playing MM cartridges through this turntable.
Recently.....I mounted an Acutex 420STR MM cartridge on the SAEC tonearm and discovered that it produced the same feedback symptoms as the LOMCs?
In experimenting with differing support methods for the Victor.....I forgot to turn the volume down before hitting the 'mute' button to 'OFF'....and an almighty noise knocked out my phono stage and tripped the protection on the left channel Halcro monoblock.
The 'Balanced' XLR inputs for the phonostage now produce no output whilst the RCA inputs produce a feeble output in the right channel only.

Any Gurus here with some ideas for the cause of this problem would be much appreciated?
Incidentally......no such problems with the 3 tonearms on the Raven AC connected to the same preamp inputs....even with LOMCs?