Ok metal, that was pretty much jibberish to me. What's a BDDI? I like cheaper. Also I don't even know what RBI/RPM are. I'm pretty stupified when it comes to this rack effects stuff.
Sorry. BDDI stands for the SansAmp Bass Driver DI preamp/direct box. Its pretty versatile and the DI sounds great, even when going straight into a mixing board. Here is a link to the BDDI on Musician's Friend:
http://bass-guitars.musiciansfriend.com/product/Tech-21-SansAmp-Bass-Driver-DI?sku=480206
And here is a link to an explanation of what a DI is really all about:
http://www.musiciansfriend.com/document?doc_id=102219
The Sans Amp RBI (which is the rackmount unit that most people are referring to in this thread) is basically the BDDI in a rackmount chassis. It also has more DI outs (one clean and one effected) and is more versatile over all. Both the RBI and the BDDI are able to get sweet dirt tones (distortion/overdrive). Geddy Lee used to use the RBI in his rig for years (two of them, actually).
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The RPM is similar to the RBI, but I believe it is intended as more of a compendium to the RBI, to expand its tonal possibilities. It is also a great stand alone unit from what I understand.
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My main reason for suggesting the BDDI pedal (or one of SansAmp's other pedals, or any other preamp pedal ala the Aguilar Tone Hammer) was that you could mess around with amp simulation (and sweet tonal possibilities) without dropping the cash on an expensive rack unit. The real key is to get comfortable with your amp first and your playing style. Once you are able to know what type of sound you are looking for (or what your sound is lacking from the tone in your head) you will be able to hone in your gear search considerably.
Hope that helps.
EDIT: Oh, and rack spaces are measured in Units. Your SWR combo has a one rack space, so it is designated as having a 1U space, or one unit of space. Easy, huh?