Why not do full and proper product reviews on the various products as has been B3D's forte up to now, but add in an appropriate comparison between competing products later? IE, don't change anything you are presently doing, but just add another dimension to the site which includes comparisons. Face it, if B3D doesn't do a comparison it won't stop the B3D readership from reading such comparisons elsewhere, which might not adhere to B3d's quality levels.
It may be the case that B3d isn't comfortable with a comparison format because they aren't sure how to proceed with it. Some suggestions might be to:
*Equalize the platforms supporting the products
*Equalize driver revisions, opting for the most recent
*Equalize Image Quality settings to whatever degree possible, explaining in depth when it isn't possible, and providing screen shot differences where possible
*Equalize driver control panel settings
*Compare through a formidable array of current and past 3D games, some of which are popular, and some of which illustrate certain points of rendering although possibly not so popular, not to mention any of B3D's own benches and whatever others it deems appropriate. DX9 benches and utilities for products which maintain DX9 support, etc.
In short, after equalizing the hardware plaftorms, and attempting to equalize image quality settings, then let the comparison begin. Done with B3D's precision, this would provide a valuable service to your readers and at the same time allow you to compare products according to an objective criteria which you'd lay down behorehand.
Further, if a manufacturer wants to start dictating terms--just politely inform them that other sources for the products are available if the manufacturer doesn't wish to engage in the process, etc. Basically, a "we'll do this with or without your help, but we wanted to provide you the opportunity for input" approach. (IMO, this is always the positions review sites should take.)
Anyway, my brief thoughts on the matter of product comparisons.