...a few busy hours have passed in which I have cleaned the RE-301 - especially the heads - and have replaced the old tape with a new one...
...I have also tried to play with the Space Echo in different places in the chain:
Theremin -> Valve EQ -> RE-301 -> AMP (Peavey KB-4) (good)
Theremin -> Valve EQ -> AMP with RE-301 inserted as external effect (not soooo good)
I also tried those chains with many different settings on the RE-301, for example echoes 1-6 (1-3 quite direct, 4-6 rather lush) with several speeds and intesities, combined with or without Chorus and/or Reverb...
...but still: the TTE delivers a much warmer, fuller and clearer sound.
That means only for my aim to find a warm (female soprano) voice for my theremin the TTE brings me a big step closer, while the Space Echo also provides a full (as opposed to 'thin'), but less warm and distinct voice and allows for some interesting 'effects' which I hope to be able to explore also during the months coming...
Asked for my preference I'd name the Tube Tape Echo without hesitation - adding that my experience with the customer support is not good (I was ignored for one week now), and that I think the tape-cassette is not a good solution, as it rattles and squeeks and it doesnt look as it will last very long. Also the mechanism to start the Echo is a gag but not thought through: to start the tape you plug in the 'out'-cable, to stop the tape you have to remove the cable again...we'll see how long the socket lasts...