Hey charliee,
That’s a great question. It depends on what speakers I’m using with S5.5 in the system. For example the Musician Audio’s Knight 1 speaker is a great two-way design that has a touch of overall warmth/fullness. Therefore, I like the synergy with the S5.5 being driven with either the Coda 07x or SPL Elector which are more neutral with this speaker that does not need more warmth. When I use the S5.5 with my NSMT Clairvoyant / System two which are more neutral/transparent then I like the touch of warmth that the tube based AricAudio Motherload adds using 6SN7 tubes. Remember, the S5.5 is brilliantly transparent and will pass on the signature of your source and preamplifier, so matching this to your speakers is based on your personal taste.
Teajay