I've been extremely happy with a pair of Nola Boxer S1 speakers in a system with a Raven Audio Blackhawk LE amp. It did for me exactly what you're describing, more fullness in the lower frequencies while maintaining a very open articulate sound in that range. They brought a kind of coherence to the music I hadn't had before. The Boxer has had a number of upgrades and you likely can find one within your price range. Here's one example of the original model, which I owned before getting the S1 model, https://www.audiogon.com/listings/lis9i1aj-accent-speaker-technology-nola-boxer-full-range
Lots of reviews online and you can check hifishark.com for listings.
Lots of reviews online and you can check hifishark.com for listings.