My solution to bass/treble tone control is to bi-amp which allows me to adjust the treble/mid and the bass independently.
I have some DIY Open Baffle speakers with a nice set of Hawthorne Audio Augie "woofers" that begged for a bit more power than any of my sub-five watt tube amps could provide, so I bi-amp such that my tweeter/mid run off my tube amp and I use a pair of Rythmik 370 OB plate amps to drive my Augies in true stereo.
These plate amps have a switchable 12/24 db low pass, a crossover frequency level, and a gain level that can be adjusted depending on preference. I have a general setting that I like "mostly" but certain recordings beg for a bit of tweaking: adjusting the gain and crossover allows some pretty major tuning, while switching the low pass from 12/24 changes the character by creating more/less of a upper shelf and adds more than a subtle character change. Combine this with a separate gain on the tube amp for the tweeter/mid and I can adjust the treble/mid quite effectively.
It can be pretty subtle too. For example, if I need a bit less high bass/low mid, I can crossover lower than normal, say 50 Hz or so and/or switch to the 24 db shelf to make a hard low pass shelf (or if I need more I crossover higher, say 125 Hz or so and/or switch to the 12 db low pass for a smoother roll off). With three distinctive adjustments on the bass and a separate gain adjustment on the treble/mid, the variables are numerous and complex indeed.
Bi-amping with the right second amp thus provides fairly detailed tone adjustment for balancing bass and treble/mid without adding any additional "stuff" in the signal path. I never really thought about it as a set of tone controls until I set it up and started using it that way quite naturally. I was just looking for a bit more punch from the Augies, if you will.
The adjustments are an enjoyable listening ritual now, particularly with a serious listening session.
Food for thought....
John
John Kessel
Hawthorne Audio AMT K2 Reference Speakers
Paramount 300B w/MQ All Nickel Iron, Mundorf S/G 5.5 uF, and Vcap Teflon .1 uF
Auralic Taurus Preamp/Auralic Vega DAC/Auralic Aries Streamer
and lots of room treatments!