Usually ...
There is a footswitch jack on any rackmount units, to bypass the effect.
If the rackmount unit doens'nt come with a footswitch, you should be able to build one for cheap. Depending on how complex the switching is on the unit of course.
If not footswitches are available to buy for most rackmount units (like a lexicon, TC Electronics, etc)...
This is how it will work :
1.) Match your D-lator's S&R w/your effects loop on your amp
2.) Put your effects(of choice) in the S&R of the D-Lator
3.) Then you should be able to midi switch the effects bypass using the footswitch output on your rack unit.
Your Line6 DD-4 should work fine with the D-lator but, I recommend TC Electronic M350" if your dough is on the low then look for deals on any old Lexicon stuff -Try Ebay or Craigslist if your lucky
Above all you need to look for a rack mount unit that can use a 100% wet signal. I am a reverb addict too!
For cabs I use and recommend any 2x12 oval ported cab...Some examples (if your not familiar) are AVATAR(for around $150-$200), Thiele (on eBay unfinished for #$100-$120), Two Rock makes a great cab loaded with W.G.S G1265 (Celestion G-12-65 clone) for about $400-$500 used.
Here's a list of speakers I have gathered in my experience-
1.) EvM-12-L (This is the most popular as Dumble himself used them!!!)
2.) Celestion G-12-65 (WGS are good clones)
3.) Emminence Tonker, Swamp Thing, ETC.
4.) Scumbacks
5.) Fane (good efficient speaker with 100+ DB rating)
6.) Early Altec Lansing (JBL) -Are good for the 1st gen. NON-HRM based amps.
Maybe someone else could add to the ones I forgot
When it comes to speakers really everyone has their own preference !
If you have any other questions I will do my best to help!