i can think of a few diferent ways a system can sound better at low levels.
in your case, going from the Simaudio ss to the tube preamp, the tubes likely have a warmer, more full character which can satisfy at lower levels, whereas the ss preamp needs to 'get going' before the body and fullness of the music come thru. ss can sound lean and light weight at low levels.
another thing can be the room or speakers or a combination of both. at lower levels room issues are minimized, both in the low frequencies and the highs. depending on what issues there might be low listening levels might avoid those issues.
in a perfect world a system and room is optimal at many different SPL's; since each recording has an ideal volume, and as our moods change so (sometimes) does the volume. if you only listen at low levels your musical choices will get much narrower.
in your case, going from the Simaudio ss to the tube preamp, the tubes likely have a warmer, more full character which can satisfy at lower levels, whereas the ss preamp needs to 'get going' before the body and fullness of the music come thru. ss can sound lean and light weight at low levels.
another thing can be the room or speakers or a combination of both. at lower levels room issues are minimized, both in the low frequencies and the highs. depending on what issues there might be low listening levels might avoid those issues.
in a perfect world a system and room is optimal at many different SPL's; since each recording has an ideal volume, and as our moods change so (sometimes) does the volume. if you only listen at low levels your musical choices will get much narrower.