Mid-fi tube vs high-end SS

"...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.

@samureyex - still not sure why you are looking at switching to SS. If you like the sound you are getting from your tube amp but are just looking to raise the quality a bit, you could consider upgrading the decoupling capacitors. 

Your amp comes with pretty cheap CDE film caps (at least they aren't electrolytics). There are a LOT of options for caps that will likely improve (and certainly change) the sound.

I really like the copper foil caps from brands like Miflex and Jupiter, but you may not have adequate space for them - they are physically significantly larger than the CDE caps. 

A reasonably priced and sized option is the VCAP ODAM. These are very clean sounding, but are also very neutral which may or may not be what you are looking for. 

If you don't feel comfortable making this kind of modification yourself, I suspect you can find a local electronics repair shop that will do this for you at a modest cost.