Perhaps the most well known Spica mod is the one made by Crosby. Crosby was a "A High End Audio Specialist" located in Hobbs, New Mexico. The modifications appear to have only been basic tweaks on the TC-50 design, but the word on the street was that it took the Spica's to the next level.
Crosby rewired the TC-50's with Randall Research cabling and incorporated a internal front-to-rear brace which you could adjust to tune the bass. An additional panel was added purportedly to the rear to stiffen the back, but this may have been to accommodate crossover changes. The internal brace must have been a good idea as a similar brace l was incorporated into the TC50i when it was launched from Spica in 1988.
There were apparently two version a basic mod and the "Strads". Information on difference between the two versions is unclear. In the higher end "Strads" the cabinet was apparently replaced with a heavier MDF and wrapped in a plastic laminate (Formica) exterior. The list price of basic Crosby mod'ed Spica's was $995.00 another $230.00 for the stands. A subwoofer was also offered for around $500.
Here are some observations of the construction of the Crosby "Strads" from speaker owner Stan Shaffer:
Cabinets are braced with an adjustable truss between the front baffle and the rear of the enclosure so they can be somewhat tuned for resonant frequency of the baffle etc. In addition, there is an additional 1" piece of MDF covering the back of the cabinet, inset by about 1/2" and the edges so it is not easily visible. The panel can be removed to access the brace adjustment. The woofer and tweeter frames and magnet structures are well damped with mortite or blutack.
Crosby also insisted on very heavy stands with the Spicas bolted through to the top plates of the stands and filled with a sand and lead combo bringing the weight to about 200 lbs per stand (They want at least 2000 psi at the spikes) My stands were made by Sound Anchor. The stands were spec'ed by John Bau for the TC-50's at 31" in height.
I use the Crosby Subwoofer which weighs well over 125 pounds and added about $500 to the total cost of the system.
Stan Shaffer www.stanshaffer.com