Alternative to my P3ESR

I agree with @big_greg on the amp pairing.  If you read the measurements section of the Strereophile review he mentions how using a tube amp could compromise the sound in the direction you’re hearing.  Basically your amp is not an optimal partner for those speakers and they’d likely do much better with a more powerful solid state amp. 

I have a pair or Proac Tablettes, similar design (sealed versions).  I have a Rega Io driving them, and this amp seems designed for these speakers - it emphasizes the mid bass, brings a fullness to it.  A lot cheaper alternative to try before swapping speakers.  Also, as others have said, since this is a near field set up you may very well be at the mercy of your inherent boundary situation, whatever it is.  Other speakers may have a similar response in the same spot.  You could try DSP.  Lastly, if you haven’t yet, try some EL34s in that amp.  

stephenjess

Based on what you are looking for (more mid-bass and bass), you'd probably need a bigger woofer. Here are some recommendations for you:

(1) Philharmonic BMR large stand mount speakers can be bought new for $1,950 to $2,100 depending on finish. They have a ceramic 6" woofer, a 2.5" mid, and a RAAL ribbon tweeter. Bass extension to 36hz. Best speaker under $2,000 I've heard, excellent mid bass and bass too

(2) Buchardt Audio S400 MkII (Not the MkI) can be found used for around $1,500. It has a 6" long stroke woofer, and a 5" x 8" passive radiator, with a .75" tweeter in a round waveguide. They claim to go down to 33hz, but I think 38hz is more realistic; still great for a small stand mount speaker......I've owned mine for about 3 years, excellent speakers

(3) Mofi SourcePoint 8s - $2,100 per pair, 8" concentric driver with 1.25" tweeter. Excellent sound, but I've only heard them twice at Axpona shows.....so I'm going off of memory

If you could stretch to $3,000

(1) Fritz Carbon 7 SE MkII - $2,900 per pair, and are brilliant speakers for the money. 7" carbon/paper woofer with 1" dome tweeter, go down to 38hz; very easy to drive as well; incredibly well thought of speakers

(2) Revival Atalante 3 - $3,100 per pair, and again are brilliant speakers....some of the best stand mounts I've heard. 7" woofer with a 1 1/8" tweeter, and claim to go to 44hz.....but they sound like they go deeper

Best of luck

I doubt you will find anything "better" than the Harbeths under $2k. Different, of course.

lack of mid bass is likely from something like a desk reflection causing the bass null...try adjusting your boundaries if you can. The Harbeth's should be strong down to around 70hz. 

Try raising the crossover point of the REL T5...that will help fill in the hole.

If you require a high subwoofer crossover point....especially above 80hz, the REL T-Zero will do better in that 80-130hz range. 

A measurement mic will really show you what's going on. You may just be a mid bass head ;-)