Valve amplifiers

Superted,
I haven't heard the Mastersound 845 amplifier but it's has a good word of mouth reputation. There are numerous highly regarded 845 SET amplifiers available. They cover a fairly broad price range. Try to make an effort to hear others and get familiar with their varying sound characteristics. Key question is what speaker would you mate to them? This is a fundamental point with SET amplifiers. Take some time to read and learn more about these types of special amplifiers. You obviously have good ears.
Charles,

The world of tube amplifiers is vast. There are certainly many other amps that are worthy of a listen!

Much depends on your loudspeakers. What kind are they? What is their impedance? How efficient are they? How large is your room? How lively is it? These are some questions that if properly answered, will help you a lot.

For example, if your speakers are 8 ohms and 91 db, and your room is average size, you will want to look at more powerful amps. That might mean something push-pull rather than SET. SETs are best used with speakers that are more efficient, such that they are never asked for more than about 20-25% of full power. If you obey this rule, you will get the best investment dollar out of an SET.

So answering the speaker issue is a must. You might also want to take a look at this link:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

The world of tube amplifiers is vast, no argument on that point. Superted was captivated by the sound of the 845 SET amp he heard and I understand this and thus stayed on subject. I'd encourage people to listen to all types of amplifiers and expand their knowledge and experience base. When all is said and done Superted may still gravitate towards the SET beauty he recently heard.
Charles,

"Am I buying in to trouble by going down the valve line?"

It all depends on:

a) how much unique pleasure you get out of using the tubes
b) reliability of equipment and support from the vendor if/when there is a problem
c) maintenance including dealing with tube bias as needed and cost of periodic tube replacements is not a concern
d) tube and SS are two different paradigms with unique requirements for best results. Any change from one to the other may well have a ripple effect initiating other changes until all is well again.

For me, I have found value using a tube pre-amp for example, but it is a very fine line that I might cross again easily. The value proposition will vary case by case.

To minimize lurking tube troubles I minimized the number of tubes in my gear (while still retaining their audio magic). CDP has one tube, pre has three, monoblocks have 4 each. Been very happy ever since.