Other thing i'd say is- it depends on what you mean by "low volume". If you mean "smaller gigs", then those lower wattage heads might be worth a look. if you mean "bedroom volume", then those lower wattage amps are going to be way too loud too- and the higher wattage high gainers turned down may not sound any worse (and may well sound better) at super-low volumes.
This reflects my own personal experience. I personally find low wattage high gain amps a bit of a strange product. I'm coming from the viewpoint of using an amp in a traditional high gain rock band with loud bass and drums and potentially another guitarist. Other scenarios may call for different equipment.
I find low wattage (5-15) watt high gain amps have a very narrow bandwidth of usage where they are the best product to use - and this is when you need a good power tube crunch with a band but not too much volume, and also you don't need a pristine clean. However beyond that volume (or if you need a clean) I'd pick a 50-100 watt head every time. You can turn a big amp down, but at some point the small amp will run out of power and sound narrow and will lose its clean. Again, this is in relation to high gain tones, other tones may require different equipment.
More watts also sounds bigger (more bass) at low volume, though how 'alive' a high watt amp sounds at low volume is down to design, as many sound like they're 'half asleep' at low volume. Many people choose to use an OD pedal to push high watt amps into life and tighten things up and this works at all volume levels, bedroom or band.
I own two high wattage high gain heads with variable wattage, a Blackstar 100 watt S104EL34, and recently scored a Laney Ironheart 120 watt for a good price (but don't have enough experience with it yet to write a review). With regards to high gain, and considering I play mostly in the home however occasionally do play out, I see no reason to not buy an amp with variable wattage (budget permitting), as you get the best of both worlds.
The Blackstar can be run from 10-100 watts, and the Laney from <1 watt - 120 watts, so you can choose your power band for the home or stage, no need to compromise. I have only used the Blackstar with a band, and I personally think the DPR works as advertised, you can dial in how much power amp drive you want at band levels.
Personally I believe modern (current) high gain is about having high pre amp gain paired with a big power amp section which stays quite clean, the further back you go in 'high gain history' the ratios change more in favour of power amp overdrive which in user terms means either more volume or less wattage. This is of course a gross generalisation. For the stuff I record I always run my amps at full wattage, even though the volume is low.
This is all in my opinion, hope it helps.
Last edited by guitarfishbay; 5th October 2012 at 08:52 PM.