[WireLine]

Depends on the amp and (strangely) the wall...If a closed back distance from backing surafaces are less critical...if an open back, a concrete or brick wall will probably cause even the most expensive amps to sound very very bad, where as a solid wood wall (a rarity, I know) will likely cause an amp to sound better than it really is...this is one reason why recording studios cost so frakking much to build, as the walls can make or break the overall sonic signature of everything picked up by mics...

That said, I try never to run speaker cabs parallel, as this can develop into the dreaded standing wave...even just an inch difference breaking the parallel surfaces will help tremendously...

Also remember the floor is just another wall, as far as sound is concerned...and wooden floors will invariably resonate and increase bass, where concrete or brick will likely reflect highs resulting in a very brittle sound...

Advise is to do like most people do anyway - have some sort of stand (even just a chair) angled up a little and make sure there are no parallel surfaces between your speaker cab and any walls or floors around, and you should be just fine...