My favourite amp has built in high & low pass filters but I've been bypassing the front end and using a tube preamp so the nerd in me thinks maybe I should add the filters. I've got a lightly used Broughton Hi&Low filter on the way.
More knobs.
And I'm curious to see if the change will be subtle or in my face.
I was surprised to discover Broughton is just one guy making stuff. He's managed to have found world wide acclaim.
I've never seen an amp with filters, but I have a lot of experience with low pass filters on Alembic basses. I also (fairly) recently installed a JTEX Distiller low pass filter on a Warwick 32'' 5 string. Really brings some character. I don't know what it is, but they really appeal to me. Wal uses a very similar circuit.
Many modern Class D amps today have both high and low pass filters , some are adjustable , and many are preset.