JT: welcome to the forum which is the greatest place Klipsch on the internet. I will go further to say that this is one of the special placed to be found on the web. Congratulations on your new to you Heresy. You know there are a lot of Heresy out there and if you feel like playing with this pair why not? You can either undo what you don't like or just get another pair. I would suggest you get a second pair. That way you have a reference point ro check your work against.
The CT125 is a very nice sounding tweeter and easily betters the stock K77 in both smoothness and extension. Given the age of your Heresy new capacitors is a very wise idea. As far as crossovers go I agree with SET12 of this forum (who happens to have an excellent thread on a Forte crossover upgrade over at audiokarma dot org) in that large gage air core inductors are the way to go, 12 gage good, ten gage better and 8 gage best. I also believe that better quality auto transformers go a long way toward better mid range. The large C-core German ones are excellent, at about $150.00 each to cheap but worth every penny. The woofer in your Heresy is ok, the K28 in an H3 is better a Forte woofer is better. bigger motor more control. The biggest problem with a Heresy is that the box is substandaed in that it is small full of air leaks and totally unbraced. Below is a link to som pictures of how I think a Heresy should be retro braced. Not that this would not be how I would brace a cabinet I designed from the ground up, this is a retro job in an already undersized cabinet. Materials must be rigid so the choice of red oak. These cabinets also have a structural brace between the back baffle and the mid driver motor not shown in these pictures. There is a lot of mod work that you can or could do to a Heresy to make it better but the question arises do you want to?
You need to ask yourself what you want to end up with and then go from there. The point is that while you can learn a lot playing with your Heresy you will spend time and money and still have a Heresy at the end of the day thouh they will be better they will have the same basic compromises of a stock Heresy.
Hear is the conclussion that I have come to after two years of playing with a bunch of Klipsch speakers which include Heresy, Heresy 3. Quartet. Forte, Forte ll, KLF20 (two pair) and CF3 parts and that is that bigger horns sound better than smaller horns do. In general three ways sound better than two ways (Heritage Vs Reference) untill you lower the crossover point of your two way to the same range as a three way at which point I prefer a two way with a pair of subs. Efficiency is important and the higher the better. At this point the Heritage line shrinks to two, La Scala and Khorn. I don't have corners and I don't want to make them and I don't like all the compromisws of the Khorn so that leaves a La Scala style cabinet. The biggest issue I see with a La Scala is the K400 mid horn. The La Scala can be turned into a very nice two way. The Peavey SP1 or the two part combo of the FH-1 and the MF1X can be bought for peanuts. The FH-1 is essentially a better La Scala and the MF1X is a half size EV HR9040 bi radial cd mid horn called a CH-1.The CH-1 comes with a good driver called a 22A which has a very cool one piece aluminum former diaphragm and a Mylar suspension which can be upgraded to a titanium diaphragm (unfortunately not a one piece design). This would give you a superb set of parts from which to start a project and at a lower cost than your typical set of Heresy. Yes they are big and some might say ugly but you won't want to switch back to a Heresy of any kind after hearing them. There are diy alternates to the mid hi section take a look at the unity and synergy horns designed by Tom Danley but for a first major project I would suggest building as little as possible. Your hands and mind will be full with lots to think about just doing a major modification. Think about it and figure out what time you have and what resourdes you have and you can go from there. Ask question as there are no shortage of great people here who have likely done what ever you can think about and more who are happy to share with you. You can have a lot of fun and learn a lot and meet a lot of amazing people along the way. What ever you want to do it is the right thing to do for you. Good luck and best regards Moray James.