Volume control for power amp

Hi and thanks for reading this post.

I have a 3-way active set up at the moment with one amp driving the tweeters, another amp driving the bass units and a quad 405-3 power amp driving the mids.

The problem is that there is too much midrange information coming through the speakers and with the quad being a power amp only there is no volume control to adjust the sensitviity.

I am thinking of soldering a resistor across the positive and negative terminals of the phono input (screened cable) of the quad in an attempt to lower the volume a little.

Do you think this is a good idea, if so what value resistor should i use.

Many thanks in advance. John.
I would install an actual volume control so you can get the setting exactly right.

I would use a 100K potentiometer for this task, especially if you plan to use a tube preamp or crossover.
you can find Rothwell 20dB, 10dB fixed value attenuator for that purpose. all you need to provide in your post is sentitivities of all of your amps and gain.
Ideally, a 3 way active crossover is required to get the proper balance btwn all three frequency ranges.
All amplifiers should be the same.
I have some home brew Usher full range speakers driven by Monarchy Audio amps. I recently added an additional pair of 8" Usher woofers in separate cabinets powered by an Emotiva XPA-200 amp. I use an attenuator (noble volume pot) to adjust the signal level feeding the Emotiva amp. I love this set up since can adjust the amount of bass I supplement to the main speakers relative to the music I'm listening to.
I would try using 1 amp first and see how it sounds.