what will make the greatest differance in sound.

Going from a ARC to a ML 380S pre-amp should certainly make your system sound different, but whether it's better or not would be strictly a matter of (your) personal preference, and of course only you can decide that. Finding ways to audition such expensive equipment (in home) as the 380S can be difficult.

Years ago Monster ICs and cables did not have a very good reputation among audiophiles. It was generally considered soft, dull, and slow with rolled off HF. I'm not familiar with their recent offerings, but that would be the first area I would be suspicious of and want to address.

Why not give the Cable Company a call (www.fatwyre.com) and discuss ICs and spkr cables with them relative to your system, and BTW, you have some nice equipment already. Several years ago I bought all my Synergistic Research wire(s) through them-- after much home auditioning, and have never looked back. They gave me about a 15% price break. The home auditioning was worth a lot. Good Luck. Craig

The least expensive upgrade is the cables. I agree with the preamp comments.

Monster cables are the weekest link. You can sell the cables and get cheaper ones and still benefit in performance or just leave as is for less. In most of my cases I'm trying to stay as newtral(lo-reactance) as possible choosing interconnects and speaker cables. To me there are very few brands: Cardas, VanDenHul and AudioQuest that are worth paying attention even for the most cheap models. Among these three VanDenHul(if proper research is made) offers the most performance over a buck spent.

I also believe that source have to be the strongest link. The one I would recommend is EMC1

I would suggest:
1) cables first
2) pre second - if you liked tubes, you should probably give serious thought to a good tube pre and/or the Audio Aero Cd player
3) source last - Levinson 390s is quite good, but I personally do not like the 390-380-336 combo - I find it quite fatiguing - so I would break it up either at source or pre... I have found Levinson dealers are usually great about home demos, so (if possible) you should consider a home audition of the full set... and see if you don't feel tired relatively quickly too...

Good luck,
Jacob

Several other considerations:
1. The listening room - are the acoustics of the rooms well suited to the speakers and their placement? Minor changes can yield excellent results.

2. Do you have a dedicated electrical circuit? Again, for small dollars, this will provide good results.

After these two things, many of the above recommendations have merit.

Happy listening, Joel