Hafler DH300 Monoblock forever?

No! Both active component boards(PC-6) of the original DH-200 will still be in place. The PC-7A(bridging circuit) is a separate, much small board, that should be located/mounted on the rear of the right heatsink, it's components facing to the rear of the amp.

There is no PC7 board, just the regulat PC6 boards like my DH-200 has. It looks like it's wired the same also, the only difference I see is the input jacks are like those you'd find to plug full size headset type plug into, not phono jacks, so the also don't have the 2.2 ohm resistor attached to them like my phono jack inputs on my DH200. I can say this much, the chassis and faceplate for rack mount, both have silk screened DH-300 on them so it may have started life as a true DH-300 and maybe someone has already reversed it and removed to PC7 board....? I'll have to get a plug like these and get soem signal into it and see what it does.

Take a look at the twisted wire pairs from the input jacks. If they are each connected directly to terminals 1 & 2 of both PC-6 boards, you should be good to go(in stereo). If it's bridged for mono operation; only the left input will be connected, and the two positive speaker terminals will be the output.

Yes, both inputs are going right up to pins 1&2 on the PC6 boards. If I convert back to regular phono jacks on the inputs I imagine there must have been some good reason for the 2.2 ohm resistor on the right input jack of my similar DH-200, between it's (-) and ground...? There isn't one on the left channel, nor was there supposed to be at least looking at my original wiring diagram. Thanks for all your help!

You are correct, concerning the 2.2ohm resistor(R45/pn 133022). There was only one provided, in the kit. The DH-200 schematic shows that on the ground of the left input. Given the age of your amps(30yrs+), you might consider obtaining some new power supply filter caps. Though yours may still work; I promise you will notice a marked difference in performance, with new ones. ( http://www.digikey.com/product-detail/en/CGS103U075V3C/338-1244-ND/953292 )Happy listening!