Speaker cable length

I have Cary tube amps and Harbeth speakers and I am kind of limited as to the placement of my components in my listening room. The optimum positioning for best sound would require a 27 foot speaker cable on one side and an 18 foot cable on the opposite side. Are these runs to long and could they harm my equipment or somehow lower sound quality?
I think these are too long. I put my power amp between the 2 speakers and run a 6 feet length of speaker wire to the speakers. The interconnect from my preamp to amp is about 16 feet...works fine. In any case, I would have equal lengths of wire for both speakers.
I also have a similar set up, but keep shorter speaker cables (6)', amp resides between speakers, and running longer ICs( 15') from amp to source.
Hope this helps.
Problem is that between where my speakers should be positioned for best listening is a large floor to ceiling window so putting my amp between speakers wouldn't work.
Depends on the cables and amp.

At CES in January 2007, AJ Conti of Basis Audio ran a pair of speakers with his cables. One speaker was wired with a 6' pair and the other speaker was wired with a 75' pair.

Not a single listener could discern the difference.

Go figure.
Your biggest enemy with long speaker runs is resistance, so if you're stuck with 27' runs, go for the heaviest gauge cable you can find (and is practical).

If you're concerned about the different lengths (27' versus 18'), then just use two 27 foot lengths and hide the extra few feet of cable behind your amp or speaker (but don't coil it up).
That said, at the speed the signal travels through the cables, you're unlikely to hear any difference at all even if you use different lengths of cable.
Lots of folks (including me) will say they would prefer something a little shorter, if nothing else than to reduce the cost of the darned cables (and I'll admit to that second part). But you already have enough of the answer above - use the heaviest gauge you can and don't worry about it (think BlueJeans 10AWG or something like that). If you don't have identical lengths that's also no biggie - electricity propagates at approx 1ft/nanosecond and you can figure out the rest (as in, it just will not matter).
Long speaker cables are not as bad as you think. Just use a low capacitance design. Otherwise it will be like hanging a capacitor over the speaker leads. The type of cable is more important than the length. The Cardas Neutral Reference remains consistant over long distances for instance. Try to keep the cable at of below 35-50PF/ft. The lower the better.
Long interconnects are better if they are an option. Jallen
There have been many posts regarding speaker vs IC lengths. Many opinions and not much science. The one fact is that the time difference between 27 feet and 18 feet of speaker cable wil roughly translate into a few millionths of a second. Your ears are probably better than mine but I couldn't hear any difference. Don't worry about it. I chose to run long shielded ICs to monos sitting beside my speakers, but that is just personal preference. Heavy gauge speaker wire lessens the resistance, so try 10 or 12 gauge. One very affordable option is TO buy Canare 4S11 4 wire speaker cable. It is effectively 11 gauge, twisted to reduce RFI and EMI, unshielded with good copper, and roughly $1.00 per foot unterminated.
I wonder why nobody mentioned inductance of the straight wire. It runs in order of 0.5uH/ft and for 27feet of cable it is 13.5uH. At the frequency of 20kHz it represent impedance of 1.7 ohm. Use even lengths for the left and right speakers.

Resistance of the cable is less important - inductor in series with the woofer has around 0.08 Ohm (limits DF to 100). For that reason don't use gauge 10. It won't help and you run into risk of frequency smear since skin effect in copper, as far as I know, starts at about gauge 20 with 20kHz frequency.
As short as possible. IMO.
Acctualy in my next speakers I am planning to run the wires from driver directly to my amp(s) and keep them under 6' total. Binding posts are a must for commercial products but my believe is: simpler the better as well as less is more. YMMV.

I had the same problem....I purchased monoblocks and had to choose between longer IC's or speaker cables. I use NBS classic series. I called Walter Fields the designer and asked him of his opinion. Without hestitation he said go with the longer speaker cables 6ft to 12ft....and keep the IC's short. Well I can't tell a difference....which is a good thing and I can arrange everything as I please.
all depends on your imput impedance. on your amp and pre amp. sometimes you your bottom end gets weak. just like if your interconnects are two long