If i have a speaker cab that can handle the volume, will it work to put a boost pedal between the speaker out and the cab's input so as to add volume with out changing tone, kind of like the opposite of an attenuator????? Has anyone done this? seems like a great idea in theory...
no, its a stupid idea in theory too.

firstly: the boost pedal would lead to an ohm mismatch. kaboom

secondly: the output of the valve junior would be too large a signal for the pedal to be able to handle. you'd fry the hell out of it.

thirdly: the output of the pedal would be too weak to drive a speaker. fail.

fourthly: Once the pedal fries/catches fire/melts, you'll have nothing going to the speaker because the dead pedal with break the circuit, and because there will be no load on the amp, it will probably blow out all the tubes at best - at worst, it will make the amp catch fire too.

Don't do it.
Rig Winter 2017:

Fender Jazzmaster/Yamaha SG1000
Boss TU-3, DS-2, CS-3, EHX small stone, Danelectro delay
Laney VC30-112 with G12H30 speaker, or Session Rockette 30 for smaller gigs
Elixir Nanoweb 11-49 strings, Dunlop Jazz III XL picks
Shure SM57 mic in front of the amp
That was kind of harsh, but he's right.
A regular boost pedal is meant to boost a guitar-level signal, not the level that comes out of your amp. That's why you need speaker cables and not instrument cables, there's a lot more power going through them.