ARC ref 3,5 OR Lamm LL2.1 for my ml2.1

I would go with the Lamm over the ARC for synergy sake. If I were to go in another direction, and try another manufacturer, I'd suggest a VAC Signature Mk IIa.

Go with the Lamm. They were designed to work with one another.

As another ML2.1 owner, I am using Herron 3. I found it much quieter then the Lamm and on par with everything else. Cost less too!

The synergy is there with either preamp (say an ARC 5SE and the LL2.1D - you do want the Deluxe version) and both are great choices. The 5SE is more resolving, has better image outlines (if you're into that) and slightly more articulation in the lowest frequencies. It also has a remote. Tonality with the LL2.1D is superb without being overly warm or rich. It sounds like a Lamm preamp which, as you know is very much in its favor. Fwiw, the LL2.1D definitely improves - and not by a small amount - with better 6DJ8 and 12AU7A tubes versus stock. Two great choices - imo you will not make a mistake with either choice.

You can use the exercise...go with the Lammie, no remote.

Dealer disclosure...

Cheers!