Alternative to my P3ESR

Have you exhausted subwoofer setup, like increasing crossover frequency and lowering gain?

This said, I was never able to get AB1 woofers to mate well with my LS3/5a's (but they had a major bump in the upper bass).

I learned years later (sold the Rogers gear in 1986) that the combo worked much better when not using the AB1's as stands and instead placing them off to the side(s).

So, maybe sub placement as well?

DeKay

You have moved the P3 closer to the wall behind them? That can help with bass, of course.

I agree with @dekay that different subs would blend differently with the P3. I am using a small B&W with good results. Last time I looked a measurements for a REL, it rolled off quickly at higher frequencies, which would be great for many applications but maybe not ideal for yours. This is nothing against REL, just a potential case of horses for courses.

These Proacs are a bit above your $2k price range but would undoubtedly flesh out the lower mids/upper bass much more and not a hard load for your amp.  The tweeters need to be at ear level though as the treble will roll off if ears are off axis by too much. 

https://www.ebay.com/itm/116989610234

Alternatively, at a lower price point these Silverline Minuets also go quite a bit deeper than your Harbeths and are also a fairly benign load for amps. 

https://www.ebay.com/itm/358063192946

Just a couple ideas FWIW, and best of luck. 

Thanks everyone, some great advice before I try swapping the Harbeths, and good luck suggestions if I do. 

@dekay makes a great point about adjusting crossover frequency and gain on your subs.  When I swap my Q7s in I have to adjust my subs accordingly.  I found that I needed to set the crossover much higher than I anticipated and am able to get them well integrated.