The ONLY model that needs the bridging module, BDG400, is the 1st gens. The way to distinguish the models is as follows....
1st gen is more/less a faded red. Almost pink. Has the Orion "Moon" with a "Star" on the top. Its also 1" narrower. Says 50 watts RMS and Hand Crafted in the USA on top, below the model.
2nd gen. MOST came digital refrence, but there are quite a few that were also competition as well. No difference between them really. Still .25 ohm stable. If I am not mistaken, this is where the EQ/MONO button 1st came into play. The 2nd gen 225s also introduced a new power supply section, so they kicked the width up 1 inch to fit it in. The power side of the 1st and 2nd gen amps look identical. Crappy plug with solid power cables. You'd notice on the top that this has the "Moon" then Orion on the left, and to the right you'd see 225 HCCA on top the line, then Dig. Ref. or Competition on bottom.
3rd gen...whole new breed. She's a deeper red. Removable top plate. Logo and writing is same as 2nd gen, but underneath 225 HCCA, it will say Competition Amp / X-Over. This also introduced the ultra nice speaker and power plugs. Rather than the crap that the 1st and 2nd gens had, this one has a molex plug that the wires can be taken off of. The 3rd gen uses the same power supply as the 2nd gen, so she's just as good for the .25 ohm stability, and no need for the BDG400 on her either.
After this...you get the 225R, the G5, and so forth, I think the quality went down after the 3rd gen. But what do I know? I'm just a random fat kid who hates RED.