Dennis Davis has posted a review of the Audio Research LS27 preamplifier ($6995) at The Audio Beat. His conclusions:
Compared to the LS26 ($5995), the LS27 is definitely a move up the spectrum, making that thousand-dollar increase in price, along with the improved power supply and more expensive parts, wholly justifiable. I expected the improvement in soundstaging solidity, but I wasn’t prepared for a jump in tonal quality and bass definition. However, within minutes of a side-by-side comparison of the two units, my listening panel was like a group of Olympic judges, declaring a clear win for the LS27 because of these improvements. Shelby Lynne’s "Just a Little Lovin’," from the CD of that name [Lost Highway 1744825], made it clear that the LS27 had tightened up the LS26's bass and refined its highs. I never considered the LS26 lacking in these areas until this comparison. Extended listening to the two units confirmed this finding, but showed it was more pronounced with some recordings, as I could not reproduce the significant differences with most material. On one of my favorite test LPs for the quality of the upper frequencies, Salvatore Accardo’s Diabolus In Musica [Deutsche Grammophon DGG 477 6492], the differences between the way in which the two units interpreted the violin, cymbals and triangle were subtle, confirming that the LS26 was no slouch in this department. But with a few modern digital recordings, such as the Shelby Lynne, the improvements in the new unit were obvious.
You can read the full review here.