I will go against popular opinion and suggest that, unlike other Harbeth models, the SHL5+ loves tubes. I also owned M30.1 and P3esr, and I agree that they preferred solid state, but not SHL5+. Not that they don’t do well with SS, but I feel there’s some synergy with tubes that makes the speakers sound so beautiful. IMO, the super tweeter can sound a little hot up top and having some tubes in the chain helps to tame that brightness in the upper registers.
I still own the SHL5+ (rotated regularly) and have tried them with the following amps
SS Integrateds: Luxman 590AXII, Hegel H390, Naim SuperNait 3, Aavik U-280
Tube Integrateds: Cronus Magnum II, VTL IT-85, Rogers 65v2, Audio Research GSi75, Willsenton R8, and Audio Hungary Qualiton a50i
Out of all the amps, the one that created the most magic with SHL5+ is the Audio Hungary Qualiton a50i. It produces 50 watts of pure class A (does not switch to AB). It really made the speakers sing in a way that no other amp could. The 50 watts for some reason sound very powerful. It had just the right balance of that tubey goodness with a more modern, crisp sound that never sounded syrupy. However, if you need to produce big bass, you’re going to need subs. Personally, I never felt that I needed them so I ended up selling my pair of REL S/510s.
A new one might be outside your range but I see two Qualiton a50is listed recently -- one on this site and the other one on the other popular marketplace that might fit the bill.
As for Cayin and Wilsenton, etc. are concerned, they are not bad per se, but I feel a speaker of the caliber of SHL5+ deserves something special. Good luck!