Several years ago I decided to see if I could successfully blend the successful sound of the Heresy with the bass extension of the Forte'. The result has been most pleasing.
Step one was to replace the Forte's original midrange horns & drivers, crossovers, and tweeters with original heresy (1980) components, and replacing the original grill cloth with a much lighter weave I purchased from Madisound.
Step 2 led to also replacing the woofers with the new Klipsch woofer now used in the Heresy III's (K28EP)....4ohm 99db....precise, fast -- flat to 40hz in the Forte I cabinet, -5db at 30 hz (wish I could measure 35 hz).
Step 3 led to replacing the K77's (EV T35's) with EV T 350's and installing new binding posts on the bases, thus by-passing the remaining mounting board from the original Forte' crossovers. As a final step I mounted each speaker on 2" spikes...from Madisound. This last step was the most noticeable on the total bottom end, finally giving the mating of the woofers, passive radiators, and cabinets the opportunity to be as precise as those wonderful K55's and T350's.
Bob Crites helped me procure some dual phase plug K55mid drivers from '80 along with the Heresy mid horns. . Michael Swallick handcrafted some absolutely stunning 8" walnut custom risers. I located both of these gentlemen and their excellent products via Ebay. If anyone wants to try this you're going to need to build mounting templates by sketching the outer edges of your current Forte mids and tweeters on hardboard, then cutting them out with a scroll saw. The mid horns must be surface mounted from the front...otherwise the K55's are just too deep to rear mount to baffle board...and the job would be too difficult anyway. The installation of the T350's was a bear. I'm now considering replacing my motorboards with 3/4 walnut veenered plywood.
PWK ..... cheers and thanks for some of best fun per $ I can enjoy.