How can I control the center channel?

[David Jackson]

I have just acquired an older Aiwa AV-D58 receiver and it has no remote. It is not a super receiver but good enough for my shop, which is where it is. I have three speakers on a long shelf (maybe 30' long or so); two KLH 4s, which are 16 ohms impedance; they are at the ends of the shelf, and one KLH 6, which is 8 ohms impedance, and somewhat smaller than the 4s are. When I play all three speakers the 6 seems the loudest; perhaps because it, being smaller, does not need as much input power as the 4s.
According to the instruction manual, which I downloaded, the center channel volume can be set at the receiver but it needs input from the remote to do it and I have no remote!
I did find a really old Radio Shack crossover/attenuator in the garage (who knows when or why I have it). It has separate terminals for bass and treble speakers; I wonder if it is safe to use it and if I can safely bridge the terminals for bass and treble.
Any thoughts?
Thanks;
David Jackson