From a distant memory I would suggest: if you like the Creeks you will love the 551; if you like the Nait then the 551 *might* be a bit polite. It's a great amp (but also look out for a 501/556 combo which is even better!)

Hope this helps!
IMHO

Main: Roon-Mac, Oracle Delphi/SME 309/Cadenza Blue, GSP Reflex C, 218, DSP5200SEU, Sky Q, Samsung 49KS7000
Office: Roon-Mac, Quad Vena 2, DALI Opticon 1
Prototype and learning: 101, M30 mk1
Next project: Vintage LS3/5A, QUAD 303