I thought the head might be dirty, but I've clean and demagnitize them after every song I do.
I didn't even catch this, because I was thinking about the DBX.
Like
fstrat76 said...I would also suggest you stop doing that and forget about it for at least a month's worth of recording.
Back in the day...way back...when I had a 4-track open reel, and I was mixing down to a stereo cassette...I would demag regularly, like if I was recording all week (on and off) I would demag at the end of that, but I would always clean the heads and tape path after every session if the tape got a lot of work, and then if the deck sat for a longer time, I would clean the heads again.
These days...with my big 2" 24-track deck, and even before when I had the 1/2" 16-track...I stopped doing those weekly or even monthly demags.
TBH...I've been using my 2" for the last couple of years now (not every week, but I've gone through a few reels of tape on it)...and I can't remember the last time I demagnetized the heads/tape path. I don't even think about it all that much, and there has been some testing done that shows it's really not needed as much as myth has it.
All that said...with a cassette deck, considering it's a multi-track, which means real, real small track widths on the tape...I would may be demag that occasionally, since everything is kinda packed in there, and with such tight tolerances....but I mean like once a month if you really use it a lot during the month.
Back to the DBX...
The reason I asked if you were using it, is because the high level off the drums may be causing the DBX to "pump"...and that's what you are hearing.
If I'm not mistaken, there is both a compression and expansion happening with it...and possibly with the very hot levels, you are making it overreact.
That's where my thinking was going when I mentioned it earlier.
You can certainly test it by recording the same hit levels with and without.
Another thing...I would not hit a cassette deck with overly hot signals anyway...because your are NOT going to get any tape "saturation/compression" BEFORE you distort the preamps....so forget that "record hot" idea altogether. Just get a healthy signal, but keep it in the safe zone.