what speakers are you driving? Have you tried simply using the 2500A by itself in biwire? with the juice it has I'd expect it to do better full range than the biamp arrangement you currently have. At one time I used dual CJ MF2100s vertically biamped - but the benefit was based on the fact it's short on power.
If it were me, I would first try the single MF2500a, and if I really thought I needed more juice - get another MF2500a and go vertical biamping. This also permits shorter speaker cable runs if you position the amps next to the speaker.
The other option would be to try tubes on the top. Many tube amps have a volume pot that permits balancing out the volume, but my attempt at this was less than fully satisfying (ie it was more trouble than it was worth).