This is sort of off topic, but last year I found an Avatar 2/12 Cab quite reasonably priced. Some lucky guy had received two for Christmas from different relatiives, and sold one for a great price.
I found it to be one of the most versatile pieces of equipmnet I own. All of my amps have extension speaker capabilites and I have used it with all of them.
My DSL 401 Marshall is a great combo, but the internal speaker actually rattles the tubes. I get a glassy tone when I use it. If I unplug the internal and use the cab it sounds amazing. I have also used my Blues junior through it and the tone just jumps up twenty notches.
You can get their budget cab with just about any speaker combo for under 400. Also they are very well made and not fibreboard like a lot of budget cabs.
The only issue you might have going the other route is matching the speaker ohms to the amp itself. A lot of combo amps have different ohm loads.
You could damage the amp.
The Blues is alright!
Guitars: 1968 Gibson SG, 2005 Gibson SG Standard, 2006 Gibson LP Classic Gold top, 2004 Epiphone Elitist LP Custom, 1996 Gibson Les Paul Standard. 2001 Epiphone Sheraton II, 2007 Epiphone G400.
Fender Strats: 1996 Fender 68 Reissue CIJ, 2008 Squier CV 50s, 2009 Squier CV 50s Tele Butterescotch Blonde
Amps: Blues Junior Special edition Jensen in Brown Tolex with Wheat front, 65 Deluxe Reverb reissue,1970 Sonax reverb by Traynor, Avatar Custom 2/12 Cabinet with Eminence Legend V1216 speakers,
2008 DSL100 Marshall Amp , Fender Super Champ XD,Fender Vibro Champ XD
Effects and Pedals: Fulltone Fulldrive II, Fulltone OCD, Fulltone Mini Deja Vibe, Fulltone Fat Boost, Dunlop Crybaby Wah, Boss DS1, Boss DD20 Giga Delay, Boss TU2 tuner, Boss BD2, Ibanez TS9 Tube screamer, Zoom 505. Radial tonebone hot british.