Threshold to Pass upgrade?

I'm thinking of upgrading my Threshold T400 to a Pass Labs amp, either an X350.5 or if I can stretch it, maybe a pair of XA100.5s (the upgrade bug strikes again). The Threshold is now probably around 15 years old. How does it compare to these current Pass Labs amps? The rest of the system is:
Oracle MkV
Grant Fidelity CD1000
Pass X1 and XONO
Talon Raven C

Thanks for any opinions.
I have a Pass Labs X350.5 and have owned a Threshold T400. The X350.5 is much better amp. I sold my T400 to a friend about 12 years ago to buy a Krell FPB 600 ("big mistake")and you could hear it right away. My friend still has the T400 and brought it over to see if it would beat the X350.5. The 350.5 is a much more subtantial amp. the T400 still sounds great, but the X350.5 has an effortless sound, better frequency extension, better sound stage, better transparancy, lower noise floor, better detail and much more authority. But what stands out the most is how much more dynamic the X350.5 compared to the T400
Pass will be better, but not $10,000 better. But if you have the money, audio is my favorite hobby to spend it on. :-)

Lots of good amps in that range, and I would first make sure you are happy with/keeping your speakers and then find the perfect match for your final speaker selection.

If you like your Threshold and you're looking for an interim solution(doesn't sound like you are), you may have Jon Soderberg upgrade your T400.
I have not heard the Threshold amps, but did own the XA100.5 (see my review) and now have the XA160.5. I would guess that the supersymmetry of the later Pass amps would contribute greatly to lowering the noise and lessening distortion for a more clean, transparent and dynamic sound. I also own the XP-20 and Xono and can tell you they make for a great combination with the XA.5 amps. Can you hear the XA100.5 in your system to compare?
Thanks for the information from everybody so far. Very helpful. Has anybody heard an upgraded T400? I'm quite curious about this option as well - I definitely find the Class A sound very attractive and would like to keep those qualities if possible. Yes I know I could spend $$ and upgrade to the XA100.5 too....