Amp / preamp for macintosh xt50

You can do that. A tube preamp would be a nice fit. Another nice option for a power amp within your budget is Odyssey Audio!
https://odysseyaudio.com/

Those speakers are a 81db low efficient speaker. You will need a power amp capable to drive them. The Van Alstine Vision Set 400 (225 watts) will do the trick. Match that with a Van Alstine preamp and you will be in the ballpark of your $4000 budget!
https://avahifi.com/