Feedback through speakers

The way you describe you shouldn't have any problems with feedback, but I'd assume that you'll need to lower your input signal. Looks like you'll need cartridge with lower output than your 10x5 have.
nd that will take care of most of feedback you're getting if not all.
I'd recommend ones that are bellow 2mv on the output for your phono.

Unless the output of the HOMC is higher than the MM, I can't see that the output level of the new cart is an issue. Are you sure that it's feedback and not a low level ground loop which only becomes audible when you have the overall gain higher? If it's feedback, then you are going to have to do something to isolate the cart/TT system. As a first test, I would temporarily move the speaker quite a bit further away from your 'table. If the hum goes away, then you can assume it is feedback (either airborne or structure-borne) and you'll need to attack that. If it does not, then it's probably a grounding issue. As a further test to make sure it's feedback, you can place the 'table itself on a pillow or other compliant isolating surface, to make sure it's not structure-borne feedback.