I wasn't talking about ultra-low watt amps, just amps with modest power, say 15-30 watts.
In any case, a lot of people don't understand the relationship between increases in power and and perceived volume. It's logarithmic. The difference between a 15 watt and, say, a 50 watt amp is not as great as you might think.
Indeed; very important.
Another significant factor is speaker efficiency. Just changing the speaker or adding additional ones can make a rather noticeable difference in SPL output.
As a front of house person, and friend to others, I can assure you that when some kid rolls his half stack into our (relatively) tiny rooms, eyes roll. A known touring band from NY came through my former venue one time and their tech rider was insane (I almost passed on the booking, but the money as too good due to shit hot act - good thing as that gig put us on the map): for the <2000 sq ft space (including kitchen, office and my studio on the side, so really not much more than 1100 sq ft) they had 6 SVTs/full stacks (2 per person) on stage + miced/DIed into the PA (they wanted more power than we had, an illegal amount actually - you could do an outdoor show with less, so we didn't bother and just didn't tell them) + folded back into the monitors (again 2 per person; promoter had to rent additional units). They were an all electronic act by the way (I think maybe a guitar on the odd song, but mostly keys, drum machines, samplers and tables full of pedals and each with a personal mini mixer feeding the 2 stacks behind them in stereo). Their setup was in some ways counter-productive, and I think mostly designed for intimidation factor (cultivating a 'loudest band' reputation etc). They probably still couldn't hear themselves that well on stage due to the phase cancellation, which reminds me: the advice I give younger bands (on the same bill as my band - I try not to when I'm sound guy; they misinterpret the motivation) is: if it's amped, keep it out of the monitors if at all possible. I see so many kids standing right in between their Ampeg (closed back, in front of the drums) and the monitor, asking for more bass in the monitor because they can't hear themselves and the sound person has asked them not to turn up the amp any more. Put the amps behind the drummer (so he can hear and to give you some distance from the speaker,) and (assuming no keys, horns etc as regards ampless instruments), nothing but vox and kick in the monitors and it'll be the best stage sound you've ever had (in that club). This does fly in the face of the current trend of keeping stage volumes as low as possible, though it doesn't require turning it to 11 either, but I have found (after trying almost all other approaches, sometimes not so willingly) that it is the best and most consistent way to make sure you can hear everything well (up until you get to stadium level with in-ear monitoring of custom personal mixes which I haven't had the pleasure to experience yet... or even just having your own sound person for the band vs the venue's guy, which I also have yet to experience). Though when one old band of mine moved up a level and played a larger venue with their own dedicated stage monitor mixer and person (located just off to the side of us, and a good competent dude who ran sound for many a former band, including this same one, at his regular gig in a smaller club), that was pretty swell too.
Anyway, to get back to the topic, if a pair of 6L6s (30ish watts) + PA ain't enough, you're doin it wrong. Hell, an 8x10, driven by a pair of 6L6s will do for bass even without the PA (up until larger venues at least).
