Cary SLI80 as just a preamp?

not sure the unit will run safely with power tubes removed

more important is why????

You just use the subwoofer outs to drive an amp but I see no way to not power the power tubes. Generally they like to be connected to loudspeakers but if you set the bias low and aren't driving the amp section it should work. Definitely not ideal but worth a test listen. 

Another idea.  If you are attempting to now use a SLI-80 integrated amp as a preamp instead, another option would be to sell the SLI-80 and apply the funds towards a second hand SLP-98 preamp. Similar value on the used market. Having owned both, the 98 is a better preamp, has two sets of Pre-Outs, one set for your main stereo amp and a 2nd set for a 2nd amp or subwoofer(s) if you need that.