External EQ/other solution?

The Schiit Loki equalizer at $149 might be what you're looking for. Has a bypass for those times when you don't need equalization.

https://www.schiit.com/products/loki 

Pulling speakers further out into the room will likely help if that’s a possibility, and it’s free!  Failing that, you could also try bass traps in the corners.  GIK gets good reviews here and their products seem relatively reasonably priced — a pair of their corner bass traps is only $270 and would likely help quite a bit as well.  I’d at least give them a call to see what they think as they reportedly are quite helpful and knowledgeable, and like speaker placement the call is free!  The virtue of both these potential solutions is they avoid inserting anything else in the signal path.  Best of luck. 

Try a set of Nobsound springs under the turntable. Use 3 springs per footer as the table is light.

Thanks a lot.

Would the Schiit Loki replace my existing preamp?

And worth getting springs for underneath the speakers too?

Place the Loki in the tape loop if ya got one. Else between the Nad and the pre.
We can’t answer "best" w/o more info.

Get it to sound it's best first by moving the speakers around.

I am a firm believer that when dealing with problems, start small and work your way up, and NO equalizers! No Rumble filters! these units rob your system of anything good its doing and adds a lot of bad. Start by treating the room, throw some pillows (try different sizes) around the room and experiment with how they change the sound. Also, try pointing the speakers (or just one) a couple degrees in or out. Make sure cables are not touching against power cords. Make sure Turntable is level and on a HEAVY solid surface. good luck.
Matt