If the sub only has one input, you will need a Y cable at the other end as well, to sum the L & R channels.No, in this case Michael's almost invariably sage and astute advice is incorrect. Doing that would result in the L & R signals to the main speakers also being summed.
If you are just using one sub, and it does not provide separate inputs for L & R, you'll have to connect to just one channel, which may represent a significant sonic compromise. A way around that would be to connect some sort of buffer stage between the pre-outs and the sub, and sum its outputs together.
If I connect it to the left or right channel, won't it create a different load across l and r channels and thus affect the sound?Not if the input impedance of the sub is far higher than the output impedance of the outputs of the integrated amp that are being used to drive it. That will be the case if the powered sub provides speaker-level inputs and is driven from the speaker-level outputs of the amp. It will USUALLY be the case, also, if the powered sub provides line-level inputs and is driven from line-level outputs of a solid state integrated amp (or preamp).
Regards,
-- Al