Have an integrated amp with only 1 subwoofer out...

Bad idea an analog graphic eq will degrade clarity  digital eq is available through roon 

Why are you adding an equalizer?  That’s just a bandaid for something that is off in your system.  Better to identify the source of the issue and address that rather than inserting another piece of equipment into the signal path.  Look at any reviewer’s system or go to any high-end audio store or audio show and you won’t find an equalizer anywhere, and there’s a very good reason for that.  Do it the right way, not with bandaids.  Just my $0.02 FWIW. 

My humble opinion is quite different.  I have a 6-band equalizer between my CD/SACD player and my integrated amp and use when I play certain discs and its wonderful!

For example, nearly all (all?) early Motown recordings have very little low end and a bit too much high end.  A judicious use of the equalizer makes Tham sound amazing!

So in my opinion/experience, the issue is the recording.  I know of no other way to address this. Do you, @soix ??

With a tape/processor loop.

it seems you might want to get an amp with better connectivity options.

Or perhaps a cheaper alternative would be to get a preamp with a tape loop and insert that between your sources and your amp.