The NAT SETs have excellent word of mouth reputation for sound and rugged built quality and should work well with your speakers. I'd look at an 845 SET such as Line Magnetic 219 and the Coincident Turbo. Both have very reasonable pricing with very good reputations for sound quality.
In need of amp upgrade suggestions
Interested in upgrading from our Simaudio Moon W-5/P-5 amp/pre power plant of 9 yrs. Have always enjoyed the authority of the W-5, and would like to find something that really excels in low amplitude transient clarity, frequency balance, and can throw an exceptional soundstage. Digital source is pretty hi-rez (Esoteric K-03), and cabling is on par as well (all JPS Aluminata). Speakers are strong in efficiency (Daedalus Athena) at around 95db. Recently auditioned a pr of Pass xa60.8's, but frankly thought the W-5 did a better presentation, less soundstage (I know, sounds crazy, but that's the way I hear it). Open to push/pull, SET, or ss, but I suspect ss. Any suggestions?