Since, in my opinion, the amplifier is by far the most underrated component in any system, I would ensure that that part was taken care of first and then match everything else to the amp.
I am in the exact opposite camp as Stehno........Oh well such is life. The pre-amp has always had the biggest impact on my sytems performance, all things being equal, except for perhaps the speakers. However, having said that either component you upgrade (amp, pre-amp) is going to have a pretty significant impact. You mention your dealer does not stock the 140 does he/she stock or have as a demo the 17 and if so will he/she let you bring it home for a demo. No disrespect intended to Stenho we all have different experiences.
I had a Magnum 88 and went to Magnum 120s and there was not a big difference. My point is, I am GUESSING the M88 is a better sounding piece than the M66 pre and therefore agree that you should get the pre first.
There is the argument that since the power amp controls the speakers you may want to upgrade that first. Especially since you have tubes and they do not always control the bass that well, however, The M88 controlled my Maggie 1.6 quite well and is a fine amp so stick with getting the new pre.
all the best,
After much listening, borrowing friends gear, and getting my friends opinions, I have to say that in my opinion and most of my friends opinions, speakers and preamps make the most sonic differences in a system. I personally was shocked that preamps sound so different!
I think the weakest link in almost any preamplifier is the volume control system.Guy at Placette Audio will tell you the same thing about volume controls on preamps.
You have great speakers.I would personally upgrade the preamp first and see what happens.