Considering Studio Instruments

Yes, it's easy to change the pattern's key. There is a simple up/down control that shows you how many half steps you are from the original key. The bass also lets you tune it within about 6 half steps (600 cents) in either direction (though it still responds to the original notes).

But also, after dragging the pattern to the MIDI track, it can easily be transposed there. It's actually a decent instrument.

We're simply looking for pre-recorded loops that can be changed key, tempo etc. without sounding awkward.

Studio Instruments is a VSTi that has some loops included. You can also play it from a midi keyboard etc. or program your own loops. If you need more loops for SI, or already have a VSTi that gives you a bass sound you like there are MIDI loops avalable from Twiddlybits http://secure.keyfax.com/mm5/merchant.mvc?Screen=PROD&Store_Code=M&Product_Code=T_BU-DLD.zip
 
Midi loops can be transposed anywhere with in the receiving VSTi range and the tempo can be varied over quite a wide range. Also there is nothing to stop you editing them, so if you have a loop in a major key but you want minor, just go into piano roll view and flatten the third.
 
The other option would be audio loops, as you looking for ease of use you would want Acid format. They would then change key automatically using Sonar markers. They would also follow the project tempo, however if you move too far away from the original tempo and pitch things will sound strange. Also it would be very difficult to edit individual notes in the pattern. One of many sources - http://www.smartloops.com/
 
It's also possible to convert WAV files to Acid format groove clips which will open up the options a lot. There is always a lot of free WAV format loops being given away in various music technology magaziens if you want to play around and see how it works before deciding if this is the way you want to go.