[Wavytone]

Forget about DSO's in an urban location. Yes you'll see a few grey fuzzies but frankly there's not much point. Better to stick to lunar & planetary, or double stars.

If you're really determined, the background sky is effectively an extended object with a spectrum that is not uniform. Modest magnification also helps darken the background to an extent, up to 1X per mm of aperture and faint objects may stand out a little better than at low powers. Consequently this where refractors around f/7-f/10, SCT's or even my Mak at f/15 can give surprisingly good views on DSO'S in urban locations, however you Los need to get your eye properly dark adapted and that's not so easy in many urban locations.

Fast focal ratios and low powers eg large aperture fast dobs, aren't much help because as you increase aperture the optimum magnification increases too, the snag being that past 200-300 mm of aperture the seeing is going to limit the maximum magnification you can use. This kind of scope is most use out in dark country skies.

There is another way to increase contrast on DSO's - light pollution filters, however you need to find one that is a good match to block the spectrum of the background affecting you most and pass the light of the things you want to see. So you need to be sure what the effect will be on certain types of object that produce strong spectral lines, eg planetaries, or nebulae emitting on the oxygen or hydrogen lines - a filter isn't much help if it blocks these too. It would be worth polling guys here to see who's using what in the way of filters.

At the extreme are narrowband filters such as OIII filters, these work well in suburban skies provided you have enough shade to get your eyes properly dark-adapted.

As an extreme case, a few months ago there was a discussion here about a scope for Sydney Observatory which has easily the worst possible sky. The general consensus seemed to be there was no point going past 30-35cm aperture and they'd probably be best using an f/10 SCT or refractor, rather than anything like a Newtonian with faster optics.