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

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.

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

@rpeluso  Yeah, this is definitely a case of different strokes for different folks and I don’t think either of us is wrong — we just handle it the way that works best for us.  For me, I don’t want to insert any extra cables and equipment into my system if not absolutely necessary.  I only listen to well-recorded music on my main system and I don’t want anything to potentially degrade the sound of those recordings on my system, and I find it hard to believe inserting an equalizer and another pair of interconnects onto the signal path doesn’t have at least some negative effect.  If I listen to less well-recorded material I’ll listen to it on a Bluetooth speaker or something similar, but there’s so much excellent well-recorded and new music via Qobuz out there I really never feel the need to listen to poorly recorded music.  One example — I’m a huge Steely Dan fan but especially their earlier stuff is not well recorded (at least digitally) so if I get the urge I’ll enjoy them on my Bluetooth speaker (Audio Engine B2) and enjoy it just fine.  Anyway, that’s what works for me and glad you’ve found a great solution that works well for you, and the world keeps spinning. 

Run one set of the L&R outputs from equalizer to the aux L&R  inputs on integrated. Run sources to equalizer. 

Why not take an RCA splitter cable with one connection to the IA and two out. One to sub; the other to the EQ?

Not sure if this will diminish the out signal, but I once used this to power two subs from a single IA out port and it seemed to work just fine.