Ayre MXRs/Thiel 3.7s

If you like the Thiel sound, I'd say keep it. And the MXR. As I mentioned, have a friend who uses that combo and it is pretty excellent.

Trading down to an Arion HS500 with a tube buffer seems illogical.

If you want a more "tubey" sound, I can think of a few options.
a. add a tube buffer between the DAC and the preamp. This is the simplest.
http://www.purityaudiodesign.com/purity%20buffer.htm

b, use a tube preamp
also easy

c. change the tubes in the Lampi DAC for a warmer/lusher tube complement. (this needs research tho). I'm sure the Lampi forums should have some discussions on different tube characteristics.

I personally find too many tubes can be a bad thing :D

I tried hooking my Ayon CD5S to my old Audio Research Ref5 preamp and the pace seemed too slow/off. I recently tried it again with the Ref5SE and it sounded better but I still preferred just one tube stage.

If you want a tube power amp that can deliver enough power to drive the Thiels, have a look at Bob Carver's Black Beauty. I know someone who just traded his Electrocompaniet Nemos (1200W into 4ohms!) for these monoblocks and he couldn't be happier. He is also another Thiel CS3.7 owner.

http://www.bobcarver.com/vta305.asp

This delivers about the same amount of power as your MXR

"Whilst I believe this to be a very good system, I just have a feeling that more tubes would make it sound better."

After reading your post a 2nd time, I've changed my mind. I would not recommend that you do anything until you figure out what it is that you want from the upgrade. Your post is just too vague. Without a better focus on what you are trying to accomplish, you're more likely to mistake than anything else.

Why? MX-Rs are outstanding amps, capable of driving anything. I and others I know have them drving Wilsons, Rockports, etc. very revealing speakers and in some cases (Wilson) difficult loads with great effect. When auditioning components, I chose the MX-Rs paired with a Nagra PL-L over an all CJ system so I doubt you'll realize an improvement.

Hi Doggiehowser,
The Ayon CD5 has its own tube preamp section. Running that into another preamp is essentially using two full preamps which is one to many.I don`t know if the Ayon`s preamp section can be bypassed to avoid this redundancy.
Charles,

The Ayon CD5S has an AMP Direct mode and a Normal mode (which assumes it would be used with a separate preamp or integrated) like their own preamps. I think Absolute Sound did a full Ayon (CDP/Pre/Power) combo review before.