I agree with the others; it is unlikely that a 56cm trek would be appropriate for a guy who is 5’8".
Having said that, if your trek 2.1 fits you well, I would start by trying to replicate the coordinates of the touch points (saddle, handlebar and pedals) on your madone. In this case, you have an excellent reference point to start with. I’d take your 2.1 into the shop and say “What do we need to do to get my madone to fit like this”. (The CX bike is a different beast and I wouldn’t use that as a reference point of comparison.)
I’ve built up a few bikes for myself and friends and in each case, I have used their current ride as a starting point as to how their bike should be built up. We have done some tweaking here and there, but overall, It’s worked out well as a way getting a fit that is familiar and comfortable.
Thanks for your insight. I agree that I wouldn’t want to use the CX bike, since it’s a cyclocross frame.
The only challenge with taking my 2.1 into my LBS with the Madone 4.7 is the 2.1 is in another state. I bought it from a LBS there and kept it at my parent’s house, so, when I visit them each year, I have something to ride (didn’t want to keep packing a bike for the plane). I didn’t have a “pro fit” for the 56 cm. I test rode a previous 56 cm 2.3 from my LBS where I live, so, I used that as a reference point for the purchase of the 2.1. I rode it every other day to get used to it before using it for IM Branson 70.3. I was very pleased with it for those 56 miles. The only adjustment I made on my own before the event was tweaking the seat post height. I never had to take it back to the shop for further tweaks.
I read how the Trek 2 series bikes have somewhat similar geometry to the Madone’s. So, when I was in the market for a carbon bike, I wanted to nearly mimic the ride of that 2.1 that I’ll use again this September for IM Branson. So, after test riding several brands, I went with the Madone. On the test ride for the 56 cm, however, I felt like I needed to move the seat forward a bit because I noted how much I rested my hands on the back of the handlebar. On the 54, I still do that occasionally, but, my hands fall naturally on the hoods.
After the pro fit, I feel like I need to move the seat further back, though. I’m just in a quandary right now, because, on the one hand, I wonder if the 56 2.1 is actually too big, or, did I really buy a Madone that is too small.