How to make sound less detailed with warmth?

Replace the amp since its dead anyway. Change the cables next (the cardas suggestion is a good one), then if that has not sufficiently fixed your problem change out the Thiels. I owned the Thiel 1.5s and they can sound heavenly if properly matched but they do lean toward the forward and bright side. Next, change out your CD player. Then change out the power cords. Then start over and do it all again. Then if you're like me, you still won't be happy, so run a couple of dedicated circuits. Then repeat this process a third and forth time until you have spent your kid's inheritance.

Move a different speaker. Thiel's are known to have a highly "analytical" sound. With Theils, that is an end unto itself. This echos my own experience with them. It's universal across the product line or at least every model I've heard.

People like 'em but where I saw them become well-known is with their "time-aligned" drivers. Which in theory sounds good and is possibly the main contributer to the Theil house sound, but I believe without that they would not have become well-known because they don't sound good at least to the majority of people I've talked with about them.

I wonder how many of the responders recommending cable changes have owned the small Thiels. My first "hi-end" speakers were a pair of Thiel 1.5s. I was immediately struck by and in love with their detail and clarity. After a few months I couldn't finish one cd, thought my ears would bleed. It's a pity as the company is an excellent company to deal with. The speaker that replaced them at that time were Vandersteen 1Cs, no fatigue and an absolute bargain at the used price.
My usual recommendation was a speaker with a fabric tweeter until I bought a pair of Revel M20s. Detail with no pain so far. There are too many good speakers out there so you should experiment.

I still believe that the dieing amp is a must.
I mean.....its days are counting. It seems like opportunity to try different amps and see what's out there. Fatigue could also be associated with non presence of any acoustic treatment. Front wall(behind the spk.) and first reflections (sides and ceiling) a must for any audio system.

I am not doubting that Thiels colored heights and overall characteristics are in fault because my inexperience with these speakers in general.

However some things can be improved with one way or the other. I would move in small steps if I was Patch 1980.
It is part of growing up. Let it happened gradually...

amp and thiel speakers are contributing to your problem. consider tubes and a different speaker. i am partial to panels, but then, you need to select a tube amp wisely.

have you considered changing the tubes in your preamp ?