I have owned two of each. No preamp - MINI TORII hands down. It will take a 2.5V source and bring it to deafening volume if you want.

The Select will play louder than a TABOO with the same source. I do not intend on TABOO bashing, just be aware that how you employ these amplifiers determines what you will reap in return. the 4 OHM speakers do fine with either amp.

I would not say the TABOO is "thin" or that you are "underdriving" it without the preamp. You can simply use the volume all the way up at times; depending upon the speakers, room configuration, etc., you might not need to use full volume. I did using one at home without a preamp, and 89 DB 8 OHM bookshelf speakers for a while. After adding the preamp it was spectacular. The LUCID MODE is unique to the TABOO, also the CCE grid modification.  I never used my TABOO with 4 OHM speakers but I am sure it would work well.

The MINI TORII is not "hard" or "etched." These terms better describe most solid state gear. You roll the tubes because you can, not to avoid a bad sound. There may be a certain sound signature you are looking for. You can dial that in with the MINI TORII which allows and invites various combinations of input, output and voltage regulator tubes; all of which can influence the sound.

Your fears/concerns are understandable, but what you fear will not come to pass. Make the right decision for you and you will be pleased with the result.