Can I use delta 66 and motu firewire 8pre together?

you cannot use 2 soundcards together at all in ASIO mode unless the cards are designed to work that way (1010's and 66's are designed to work that way)

but if you use WDM driver mode you can use the motu and the 66 together if you use the SPDIF between them.  so you'll only get 2 extra inputs that way (running the SPDIF from the D66 into the MOTU).

if you try to use them independently of the SPDIF you will have sync problems - they will not sync together because they have separate clocks.

ooops, I was thinking of the 828, not the 8pre.

in that case, no, you can't use them together without having clock sync problems.

Can I use my delta 66 and my motu firewire 8pre together? Has anyone done this? As I recall I tried it once and had problems. Do I have to set one of them slaved to the other or something?

 
You could get an ADA8000 (well under $200) and use that together with the 8-Pre (via lightpipe).
That would give you up to 16 input channels... all with preamps.

Back to the ada8000. how are the preamps?

 
I know...  
The ADA8000 is actually a decent product.
The preamps are cleaner than the original Presonus Digimax...