Old full range speaker as new subwoofer

I listen to music at a desk with the desk speakers set 5 feet apart. The speakers are Linn Kans (70Hz - 20kHz. +/-3dB). I want more bass and have considered installing a large, very long throw speaker over my head -- infinite baffle would be my attic. But I had another thought and wondered what the forum would think of it.

I have a vintage pair of McIntosh MLC1s which still sound good but are too big and bulky to set up in my preferred nearfield listening space. Each speaker has four drivers one of which is a 12" woofer (Fs=24.6Hz). Roger Russell, the designer, set the crossover for that woofer at 250Hz.

I can put the speakers under the desk in line with the desktops. But if I play them full range along with the desktops and even if I can get the balance right, I think localization and soundstage will be severely disturbed and not in a good way.

So what about using an active crossover (low pass) and separate amp to send the speakers only signals below say 100Hz. Anyone see problems?
It will probably not sound good full range but I can't think of any reason trying using the active crossover. It may not sound as good as you recall with only 100 cycles and less playing but why not see if it works for you. I assume you have access to the crossover. Even if you don't there is no law against trying it full range just to see how it sounds.
Why not just pick up an inexpensive subwoofer... I picked up a Pinnacle for $80... Plays fine down to 35hz... I bet there are several on Craigslist this week, You'll get something decent for $100 or less.
Disconnect the wires on all the speaker drivers except the woofer and listen to what that sounds like.
@ Yogiboy -- But will your solution answer the question of whether the acoustic suspension basses-only speakers can be integrated with the desktop bass-reflex full range speakers.

Due to different ranges and sensitivities of the two different types of speakers it seems to me you'd need control of frequency range and slope roll-off plus control of volume to determine whether the 12" woofers can be made to play nice with the desktops.

@Timlub -- I prefer stereo subs and putting a matched pair together off used equipment sites is a hassle.
Check out Dayton Audio Sub-800 at Parts Express. They are
only $89.00 each. You can get two of them for under $200. I
think they would work fine for your application.