Hi Vinay, and Welcome.

That's a nice looking amp you have. Congratulations.
I also have a 300B amp and I find that it does very well with a passive transformer volume control (tvc) because there is so much gain in the amp. I have not heard the Airtight, but your amp's input sensitivity looks to be about the same as mine, so I thought I would suggest it. Too much gain can give a confused, or congested sound, and maybe that is what you are experiencing with an active pre-amp.
Have you tried the direct input feature on your amp yet?
If yes, is it loud enough but just not dynamic sounding? (lack of force and drive, or wimpy sounding)
What are your source components?