Audio Research

 This is very curious. I owned an LS2 mk2 for a number of years in the mid to late 1990s. The B stands for balanced input.  It was good for it's time but it was never great. The LS25 mk2 I had after was in a totally different league. Just so much better in every way. Far more detailed and holographic without sounding thin. 

 I agree that the LS27 is not that great.. I had a chance to buy a demo unit at a great price and passed. It sounded thin and lifeless in my (all ARC) system compared to my LS25 mk2. Audio Research has had some real winners but then there are others that are just so so. 

I like my Audio Research LS25 Mk II and think it is a real gem, but I'd be wary of downrating the LS27 (which I have not heard) without ensuring the tubes were good, it had been warmed up, and that everything else was the same in the system.  Then I'd do some blind testing...

I do have a couple of ergonomic quibbles on the MkII I own: 
- the remote is balky and a new one is expensive
- I like the old knobs best (not the earliest black ones) - this unit has knobs that _look_ like the early (1970s) ones, but they are really just twist and hold things.  I understand why they did that, but part of an expensive piece of audio equipment is ergonomics & looks.  Those contribute to the aesthetic experience, which after all, is what you are buying.
- this unit lacks an hour meter for the time on the tubes which some newer ARC gear has

I DO like the toggle switches - much nicer than the push buttons on newer units.

A good pre-amp will be a lot  more rewarding that some woo-woo cable changes.

And BTW, serious audiophile equipment should use XLR balanced cables - the ARC does of course.

 The preamp is the heart of your system. The most important piece of electronics. It has the most affect on the sound of your system.

 I had the LS27 in my system for a week. It was a dealer demo, well broken in and in good order.

 I like the looks of the LS25 much better than later Audio Research preamps. There is absolutely no need for a display on a preamp. You can't see anything on it from most listening positions. Except of course the large volume number. And ARC says you should turn off the display while listening for the best sound. The little green LEDs were much easier to read from a distance and more elegant looking..

 I also like the thin remote of the LS25 mk2 better than the later plastic ones. The later plastic ones just look and feel cheap. My REF 5se has a metal remote which is really the same remote in a metal case. I think ARC no longer has replacement remotes for the LS25 models.

 I also like the toggle switches on the LS25 much better than the plastic buttons. They are just ugly. At least the Ref 5se has the redesigned metal buttons.

 All the earlier ARC components lacked the hours meter. This is really a nice feature. Especially on amps where knowing tube hours are extremely useful.

 Audio research hit a home run with the LS25 mk2. It is a real gem. Much better than the original LS25 which I have also had in my system for an extended period of time. But the Ref 5se is just amazing. I was so impressed with the sound, that I replaced my phono preamp with the match Ref Phono 2se. A magical combination to say the least.

Have you tried a Shunyata power cord on your LS25 mk2? I have a Shunyata Zitron Alpha HC on my Ref 5se and it made a significant difference. It did not change the sound of my Ref 5se but reinforced it. 

I liked the original Audience power chord on both the LS15 and 25mkll. It's quite inexpensive now and a nice upgrade from the already good ARC cord