Cubase 6 64bit Alesis io4 only works at 384 and 88

[Scott]

Hi I am using the Alesis io4 with ASIO4ALL as Alesis recommend. I am using Cubase 6.5 64bit and it does work although I can only select 88ms (which gets some dropouts) or 384 ms which doesn't get dropouts but the latency is a bit higher than I'd like. I can set the latency to any length in other programs -Reason, Guitar Rig standalone - but in Cubase only those two seem to work. Ive tried fiddling with the other settings like forcing 48kHz etc. but to no avail. Is this something which can be explained?
I am using the beta version now but was using the 2.10 version until that and both are the same.

Thanks

Scott