mid horn driver advice

I have Jensen Imperials that I enjoy greatly (with 18" DIY subs, 5 amps actively crossed when all is working). One of my mid horns gave up the ghost. I bought some Great Plains Audio 909 and paired them with Altec 511B. Oddly, compared to the remaining Jensen RP201 horn, the MUCH larger Altec/GPA has significantly less presence in its lower range, say 500hz-800hz. They are almost unlistenable if I cross to them below 1khz . I haven’t actually tested response with signal generator and mic, but it’s pretty easy to make relative evaluation since all drivers are actively crossed over. I put the GPA drivers on the Jensen horns and the result was similar. These were my first "vintage" speakers, so I have very little actual experience with horns. I’d appreciate your suggestions. Which (relatively inexpensive) drivers might be better suited than these GPA? Or if anyone knows where to get a replacement diaphragm for RP201 that’d be swell...