Thanks for the responses, sorry, if i'm a little confused still.
Dunedin, is this reading correct?
A good pair of studio monitors would work Ok for my wifi's stereo system (Not as good as equivalent cost Hi-Fi speakers but potentially better than her current set of speakers which cost £300), and I would be able to plug in the Helix to enable me to get accurate cab simulations.
However, a powercab would not sound great for her audio system and a PA like the DXR10 is massive overkill and would probably, literally, bring the house down (It is over 100 years old and as with many houses in the area, cheaply built at the time to service people working in the now closed steel works).
I do use a set of small studio monitors when I do recording or create backing tracks on cubase (A pair of IK Multimedia iLoud MTMs) and they do sound very clean to me but they don't really fill a large room in the way you would want from a set of stereo speakers (I tried carrying them downstairs and plugging her stereo into them and it didn't really work unless you were in the right spot). Are there some studio monitors that fill the room better and that would allow for accurate cab simulations? The budget is quite high (so around the price of the powercab 212 plus or lower).
And just for those worried I have talked it over with her really. Her position is that I can do it if the speakers are better than her current ones.