Hi all,
I've had a BG250 since they first came out - I have a Mk1 BG250, which only came with a 15" speaker.
The reason they don't have an option for an extension cab on the larger combos is (as you've probably guessed) the amp is already working at 4 ohms.
There have been quite a few reported electronic failures for these (releasing the magic smoke), but at the same time there's been a lot of units sold. 1% of 10,000 units, and all that.
I've regularly gigged this combo for all sorts of styles - Rock, Rhythm and Blues, Jazz/Funk - and it's always performed well. However, it's probably worth saying that I flightcased this right at the start. These combos aren't made of glass, but they're not tanks either. I do feel that some of the problems people experience might be related to the relative fragility of the components. It is noticeable that PCBs are clearly visible through the rear vent panel.
If I were to criticise, there is a clear voicing to the combo, leaning towards to boomy bottom end. It works great for flatwounds and RnB, but less so for slappy or 'hi-fi' tones. Saying that, the diaphragm tweeter is effective, and much less harsh than bullet or horn options. The EQ section is not as it seems, and each knob controls more than one frequency. I originally liked this, but as I've gone on I've felt less able to accurately control the tone. The volume and gain controls (like other TC stuff) do not feel very linear. I suspect that at 60% volume, you're getting 75% of the power.
In closing, I would still recommend the combo (knowing the sonic limitations), but also recommend buying a good cover.