I can confirm first-hand just about all the points that Sean makes regarding the vintage Klipsch. My LaScala's have improved significantly with modifications Sean mentions. The only thing I have not done is rebuild the bass cabinet, and that is the only area I feel the speakers are currently lacking (bass currently takes a nose dive at 50hz). Here's my take on the improvements I've done:
Internal wiring harness: Replaced with DH Labs silver wires - a noticeable difference in clarity and detail. The stock harness is simple 16 ga. copper wires.
Swapped out the stock tweeter with Fane 5020 - this made a HUGE difference in the upper register bringing out details I'd never heard on these speakers before, making the entire range sounded more coherent, and musical. These tweeters required padding down as their 110db sensitivity is too hot as-is. Of the modifications I've done, this one certainly made the most significant difference to me.
Replaced stock AA crossover with ALK's crossover. Here's where I'd take some exception to this as a 'rule' for guaranteed improvement. In my much smaller listening room at home I preferred the 'softer' and more dimensional sound of the stock AA crossovers. Though the ALK's exhibited more clarity and focus, and were imediately recognizable as an audible difference, I still liked the AA's better in this space. I tried all different settings with the ALK's and swapped back and forth for many days and sessions. I ended up sticking with the AA's as they just sounded more dimensional and natural to me. It reminded me of the difference between a 'tubey' sound and an SS sound, the later being the ALK. Later I moved my LaScala's to a much larger listening room (my work space) where they had room to breath. Here in this space the ALK's won hands down delivering both laser focus, clarity and the width and breadth of soundstage they seemed not to in the smaller space. I think perhaps there was some element of 'fatigue' in the smaller space that just doesn't occur in the larger one with the ALK's. The soundstage seemed to equal that of the AA's in the larger space, whereas in the smaller space the AA's seemed to provide a wider and deeper stage. I'll leave that one to Sean to try to explain as I was baffled.
I have no doubt the bass could be improved as well, and the cabinet is merely adequate in construction (as far as speaker cabinets are concerned) and stands for much improvement in reinforcement to assist in this realm.
Still, the clarity, the transparency and the speed of these speakers in my system, to my ears, has kept me spoiled in a sense as everything else I listen to falls short somehow. Granted, this is only a personal preference, and I certainly have not heard all there is to hear.
Anyway, good advice by Sean!
Marco