Same here Raymonda, never listened to the Thorens TD 150 so I cannot comment on its sound qualities. Seems to have been well engineered and very similar to the Sondek LP12 in appearance.
I'm curious as to which four? I'm guessing RP6, Traveler, Concept, and Xperience?
A friend of mine has the new Satellite and it sounds MUCH better than the entry level Comet and it can hold its ground against much more expensive TTs.
Marantz has a quiet, steady, reliable motor, a more than decent tonearm, and KILLER cart for the money.
The WOW is in another league, comparable to TTs costing much more and almost as good as Well Tempered Amadeus and Townshend Rock 7. Dead quiet, rock solid.
Upgrading the tonearm and armboard and adding the MH CC2 sounds like the right way to go if you decide to keep the Thorens.
"purchasing many of them may well be a down grade". Not sure if I completely agree with your comment. As good as the Thorens may have been 44 years ago, new advance in technology may (or may not) place some of the models mentioned over the Thorens.
Maybe you're right, who knows :-)