Audio Research VT100 MK1 and LS15 HELP!!!

Also speakers are rated at 8ohms though

As I stated before, KEF may rate the speaker as a 8 ohm nominal load, but you look at the impedance curve to determine which tap to use (to start with).  The frequencies in the bass and mid-bass range are at approximately 4 ohms. This is the critical area of the frequency spectrum because it is harder to drive the speaker to reproduce good low-end response.

Well I did a little on line reading regarding your speakers which causes me to suspect the combination of your ARC stuff and the KEF may be your problem. Your speakers seem to be considered ’neutral speakers’ (at best). You seemed to like them when driven by a (very) warm vintage receiver). You are hearing the results of you amplification choices quite clearly. You seem to like a warmer toned system’s sound but with better resolution than you get with the old Mac.. Very common in audio.

Now you change to electronic’s which, tonally speaking, might be voiced to be neutral (at best, IMHO, very cool). So you are doubling down, so to speak, and get the sound inherent in both. For you this sound is not synergistic in that it doesn’t produce the tone you want, not that either the electronics or the speakers are wrong, they just don’t sound as you would like when combined. I would suggest that most of us have been there and done that. I certainly have. That is why most folks will recommend ’matching’ your speaker and electronics. Arbitrary buying will not get you there.

BTW, regarding 4 ohm taps. I’m a bit put off by KEF’s ’recommendations re using only the 8 ohm taps. I could understand why they think your speakers might sound best a 8 ohms (perhaps they think the 4 ohm tap might unbalance their preferred sound) but I know of no reason you should not try them. Your speakers may have have a nominal 8ohm rating but this is far from indicating that they have a minimal 8ohm ’curve. Reviews point out that they have a minimal ohm droop to 3.8 ohms. Were I you, I would research this issue before I just walked away from it based on someone from KEF’s opinion. I like warm toned systems and have most always operated off 4ohm taps.

Very well stated, @newbee 

On the rear of the VT100 mki there is a place for inserting jumpers to change the inputs from balanced to SE.  Was the seller using balanced or SE Interconnects?  

You can download the VT100 manuals on the Audio Research site.  The manual shows where the jumpers are.