Best Integrated Amp/Preamp with Full 3-Band EQ for Jensen LS-6B Speakers? Budget $3.5K 

most Marantz and Sansui amps from the 70s.

I should mention that vintage gear can be prone to needing repairs.  If you go that route, maybe find one that's had the capacitors replaced, etc.  I only pulled the one from eBay as an example of a receiver with an EQ.

I'm not familiar with those speakers, but a little Googling seems to indicate that they weren't real high end speakers.  My concern is that you'd spend $3500 and not be happy with the results.  What did your father use for an amp?  Have you heard the speakers?  Maybe they're great speakers.  IDK. 

Marantz made some cool vintage receivers that might be appropriate.  Built in EQ.

https://www.ebay.com/itm/326061942194?chn=ps&norover=1&mkevt=1&mkrid=711-166974-028196-7&mkcid=2&mkscid=101&itemid=326061942194&targetid=2282220675791&device=c&mktype=pla&googleloc=9023230&poi=&campaignid=20781933317&mkgroupid=160110931616&rlsatarget=pla-2282220675791&abcId=9339527&merchantid=137736004&geoid=9023230&gad_source=1&gclid=EAIaIQobChMI8Z6_mP2viQMVtUT_AR24Gjn4EAQYASABEgL1vfD_BwE

You are better served looking at a modern integrated amp with a dac and using roon  as roon has a built in parametric equalizer built in 

And  a parametric eq done digitally is much more transparent  then trying to eq in the analog domain

Dave and Troy

AUDIO INTELLECT NJ

Roon experts