boody & tunetown have some great advice on this matter. The key lies in subtle use of delays or early-reflections, along with the right gain/EQ adjustments. If you are not careful with your use of reverb, you can quickly muddy up a mix and make it seem less 3-D. You might want to experiment with only having 1 or 2 elements being sent to a big verb, pushing those to the back. EQ out some top and bottom on these elements, and lower the gain until they seem far back enough in the mix. For the other sources, rely on short delays or ERs.
Dreamverb has a great ER section, that can be used without activating the tail. This is a great tool for positioning instruments inside a space. Since you are using DFHS, you already have access to an ambient room mic sound. Processing this room mic with EQ/compression will yield much better results than using an artificual reverb for drums (in my experience). Setup a dedicated drum/percussion verb that you can send your snare/toms to. Try timing the decay and pre-delay to match the tempo of your song. Also play with the \"Amb Corr\" setting in DFHS; it adjusts the delay of the ambient mic signal, which can be used to make the drum room sound tighter or larger and deeper.
How are you micing your Marshall? It is usually easy for me to get a great guitar sound using one close mic (SM57 or MD421, or both) mixed with a room mic (I miss having a Royer 121 for this
). For the room mic, a figure-8 pattern works well for giving the guitar sound some width. If you have a relatively dry guitar sound, try panning it to one side and have a short delay/verb panned to the opposite side. Mix this delayed signal relatively low in volume, and play with the delay time to acheive the width/depth you are looking for.
Want more width on your bass? Try sending the mono bass signal to two tracks, and pan them slightly L/R, leaving your kick drum in the center. Alternatively, setup a send with a HPF around 150-300Hz feeding a chorus. This way the chorus will ignore the lower frequencies which do not have many directional cues, but it will spread out the upper frequencies in the bass.
For vocals, experiment a lot with reverbs and delays. Even in harder rock songs where you don't want lush/deep vocals, subtle useage will make the vocal more \"3-D\". Try using 2 reverbs; 1 for ambience or ERs, and one with a smooth tail. Vary the ratio between the 2 verbs to get the depth you want. You can automate these sends too, so that the vocals are more up-front in certain parts. Also try using a simple slap-back delay on the vocals, or automating a longer delay send, adding echo to the end of phrases or on key words. Another fun thing to try is having the vocal delay (mono) slightly to the R, and the vocal verb (mono) slightly to the L.
If you need certain things to really stand out and be up front, don't apply ANY verb to them. Try being more aggressive with compression and use additive EQ somewhere between 2.5kHz to 6kHz to increase the presence. EQ even higher up (8-12kHz) if you want something to be extra crisp.
Hope some of this is helpful...