do tube amps have weak midbass?

This will vary from amp to amp and amp to speaker interface. I would say that most tube amps exhibit the opposite characteristics of what you are describing though. Tubes are known for a slightly lush or "plump" mid-bass, adding warmth and "bloom". Between this aspect of their performance, their liquid midrange and soft airy treble, you've got just about everything that tubes are known for. Not necessarily accurate but quite pleasing to listen to i.e. high on the "musicality factor". Sean
>

My suggestion is to experiment with different speaker cables first, which can make a huge difference in tube amps.
Consult with the specialist who can recommend you a good match. Tube amps do not drow high current into the speaker wires and length does matter.

Connections in this case might play a vast part and have to be checked and cleaned... Oxided connection has an extra mutual capacity and inductance. You might also try to re-terminate with better connectors or replace with better binding posts.

Another way to explain dissipation of mid-bass is transformer influence: in the tube amps it may induce parasite currents of opposite phase and relatively large amplitude (worst case will be in-phase). This case cannot be possible for VK-75 that I consider the to be one of the best engineered tube amps on the market.

I listened to a YBA 2alpha power amp (ss) once and that had the strongest most kicking midbass I've ever heard. This was confirmed in reviews of various YBA equipment in Stereophile.
My Counterpoint hybrid amp is wonderfully liquid but lacks in this area as well, rock and roll just doesn't really work too well thru most speakers.
it might have to do with the sensitivity of the speakers as well. I heard the VK-60s with Revel Salons at a dealer (probably 86 dB sensitive) and midbass sharpness was not among this combo's strengths. Most other aspects were impressive however, as I'm sure you find with your '75s.

update:

using a test CD, i determined that the bass suckout occurs between about 45 and 65hz. what's weird is that there's decent bass energy at 30-35hz and even some, albeit greatly reduced, at 25hz. at 70hz and above, everything's ok. is this a room effect?

any thoughts?

IMO,this type of problem is almost always a product of room acoustics.