Best Bluetooth Transmitter for Vintage McIntosh?

One issue with Bluetooth is that it is limited two about thirty feet give or take. Walls and obstacles will affect this as well. I’m thinking wifi may be a better option.

sonicjoy, my audioengine goes much further than that.  They claim up to 100 feet and I agree.
Submariner-normally I would say that most of your effort for quality is limited by using bluetooth, which is actually ok.  The real limitation is XM.  In my humble opinion XM is worse than any mp3 I've ever heard.  Try streaming from internet, radio or HD radio and enjoy the difference.

Thank-you, Elevick.
I have noticed  less than desirable sound quality and stereo separation issues - especially with some older, 'digitally re-mastered' classic recordings coming through XM Radio.  Nevertheless, I appreciate the tip about Audioengine's sonic joy.  I had been leaning toward the Miccus RTX 2.0...

My Miccus Home RTX 2.0 works great with both television and other audio. I haven't tried it with XM Radio, but it works long range in my home. I get signal all over. I use the Miccus SR-71 Stealth headphones with it. Make sure you're not using Bluetooth devices next to a WIFI router, repeater, or mesh network as these can disrupt the signal. 

Many thanks, cbgrizzle.
I appreciate your sharing your experiences with the Miccus, as well as the tip regarding a WIFI router, repeater, or mesh network(s) proximity to the X-mitter.
It's a learning experience, for sure.  Most of these devices come listed as receivers.  Then, after reading their tech stuff, you learn that the same device is also a transmitter.  Some have a switch you flip, while others do not.  Some will give you stereo sound, using two speakers for the usual placement, and others are only mono.
I like antennae and I also like two RCA Phono Jacks versus using the 'Y' or adapter, though.
These devices are obscure in the sense that you glean a little more knowledge each time you visit a different brand.  Speaking of brands...
Different search criteria yield entirely different brands - some of which, despite much searching, never appeared before.
Crutchfield has been most helpful.  Did you know that in addition to the mainstream Miccus and Audioengine, Cambridge Audio, Harmon Kardon, Trond, Etekcity, Avantree, Aluratek, and Soundcast make one or more?  There are brands from England, too.  Auris Blume is yet another one that I just found among my stack of notes.