I own a pair of the Tonian Labs TLD1 speakers and have heard the Merlin's numerous times. I have to say they offer up quite a different sound. The Tonian's are fast, detailed, and more transparent. Sometimes a bit in your face. When in use I use no toe-in with them. The Merlin's, while not as transparent, are smoother in comparison and IMO would be easier for most people to live with long term.
Aside from that, Wilsynet mentioned a combination of the Lightspeed attenuator and Music Reference RM-10 MkII. I also use this combination and another Audiogon member uses this same combination with Merlin speakers and loves it. The only concern wold be the output impedance on the Herron which is a tad high at 500 ohms. The Lightspeed usually likes to see an output impedance of less than 200 ohms. You could check with the manufacturer to verify that. In lieu of the Lightspeed a buffered preamp like THE TRUTH of Pass B1 would work well.
Aside from that, Wilsynet mentioned a combination of the Lightspeed attenuator and Music Reference RM-10 MkII. I also use this combination and another Audiogon member uses this same combination with Merlin speakers and loves it. The only concern wold be the output impedance on the Herron which is a tad high at 500 ohms. The Lightspeed usually likes to see an output impedance of less than 200 ohms. You could check with the manufacturer to verify that. In lieu of the Lightspeed a buffered preamp like THE TRUTH of Pass B1 would work well.