Mid-fi tube vs high-end SS

The degree of audio performance improvements with a new solid state amp you are hoping for is entirely dependent on the synergy (or lack of ) with your system as a whole. One new piece in the link may or may not be an improvement.

For example, in a prior system, my buddy asked to borrow my speakers at that prior time ( TOTEM FORESTs) paired with my quality build ss integrated amp . He took them home to test-drive them with his top-end CARY tube amp. 

In short, it was a huge disappointment and a step back in his system as a clear and not insignificant absence of system synergy.

Ignore the pricepoint hyperbole trap,  and rely on your personal audition(s) to drive your selection process. It is impossible to blindly recommend an amp brand or particular model for you with any assurance of an improvement for you, without a direct knowledge and experience with the rest of your system. 

Choose wisely.

"...how much improvement should I expect?"

"Improvement" is probably not the right word.  What you can be sure of is that a solid state amp will sound "different" than your current amp.  Whether or not that will be an improvement is a matter of personal taste. SS amps typically have better control of the bass, but behave differently when they clip (which is one of the reasons for the trend toward high power in SS amps.)  Or, you may find you miss that rich "tube sound" that so many tube aficionados love.

In short, pick a likely SS candidate that is available for a home trial and can be returned if you don't care for it and see what you think.

what efficiency speakers are you using?

contemplating less efficient speakers in the future?

What sound levels, how large a space?

Think 'prefered', not better.

IOW, I would expect to prefer the sound of tubes over sound of SS, unless not enough power.

Another thought is speakers that can be bi-amped, use SS for Bass only, keep tubes for mid/highs

My speakers are the Philharmonic BMR towers. 6-ohm 86 db. Easy to drive, confirmed by the designer. My tube amp drives it without a sweat.

I feel like the midrange on the BMR is too neutral. The designed voiced the speaker that way. My preference I want a little more richness to the mids and I'm getting that from the tubes. 

AHB2 is a no-go for me for 2 reasons. Price has gone up from 3k to $3,300 and also its weaknesses are things I value highly. 

Thank you everyone. 

I know you’ve done the tube "upgrade", but have you done much experimenting with other tubes? There are lots of subtle difference between tubes, and some not so subtle.  You may stumble into something you prefer that may not be the recommended upgrade, but it’s all very subjective.