I played a very similar setup for many years. I played a 1966 Fender Jazz into a 120 watt Yamaha head through a single 15". We even played material that was very similar to what you are describing. That setup sounded fine. In fact, I met quite a few bass players who praised my sound. I had the volume on about 8 or 9 and the "tube distortion" knob on about 6 or 7. Pretty distorted. And, headroom? What headroom? The head was basically running flat out. Remember that you only need to worry about having a lot of headroom if you are wanting to play clean. No amp cracked to the max has any headroom. So what? It worked fine. Had a very "old school" sound, which was perfect for the classic rock stuff we were doing. If you were going for a cleaner sound, then you
would want more power to give you more headroom. But a single 15" with a 200 watt head should do FINE for the kind of clubs you've described and the type of material you're doing.
There are a lot of myths being propagated here. First of all, the old claim that "tube watts sound louder than transistor watts is just plain false. A watt is a watt is a watt. Repeat it to yourself until it sinks in.
As for the claim that bass players need three to four times the wattage as the guitar players, well, this is sorta kinda true, but only as a VERY rough rule of thumb, not a hard and fast rule. There are simply too many important variables that come into play when it comes to how loud a particular amp actually sounds for that to be of much use. We've already discussed that it matters if you're going for a "clean" tone or one that is heavily overdriven. But there are many other factors, too. For one thing, the tonal characteristics of the amp will have a BIG influence on how loud it sounds. An amp that has a lot of midrange will sound louder, all other things being equal, than an amp that produces the same volume on a db meter, but has a more even frequency response. Look up the "Fletcher Munson Curve" for more information on this phenomenon. Also, the amount of distortion present in a signal will alter how loud it appears to be. If you are a sound man at a big arena and you solo the piano (which, we will assume is being run through a super clean preamp) and set the level to a certain level using a decibel meter, and then solo the lead guitar and bring it up to the same level on the meter, the guitar will SOUND much louder to you, although the meter will read the same. There is another psychoacoustic phenomenon at work here. The human ear simply interprets distortion as meaning "this is loud."
But by FAR the most important factor in the loudness of a particular amp is the efficiency rating of the speakers. Remember that doubling your amplification power only gives you about 3 db increase in volume. That means that, (again, if all other factors are equal) an amplifier with 200 watts into a speaker cabinet rated at 103 db efficiency will sound as loud as a 400 watt amp into a cabinet with a 100 db efficiency rating. And there can be much more than 3 db difference in speaker cabinets!
We would really be better off if we just dropped the whole notion of thinking that we can tell how loud an instrument amp can be just by quoting the wattage. There are just too many variables at work in instrument amplifier design for this to be valid. It's not like with power amps, where the frequency response (flat) and the distortion ratings (minimal) are held relatively constant. In THAT situation, wattage
does have some practical meaning. But it's a very different situation when describing instrument amplifiers. There are just too many OTHER factors at work for it to be very meaningful.
If you have decided to go with a 300-400 watt amp instead of the 200 watt amp that you've got your eye on, simply because of the posts that you've read hear, then I'm afraid that you've been misled my friend. Remember that going from 200 to 400 watts will only result in 3db increase in volume, even if everything else is the same. (And they won't be, of course.) That is too insignificant an increase to mean anything in practical terms. It would mean the difference of about one or maybe one-half notches on the volume knob. (i.e. If we're going to say that the 200 watt amp goes to "10," then the 400 watt amp goes to about 10 1/2 or 11.) If you want to double the volume level, you would need TEN TIMES the power to accomplish this.
Bottom line - if you like that amp you've found, and you like the tone it gives you, then buy it and don't look back. As has been pointed out here, already, your guitar players may have MORE amplification than they need for the small clubs and cafe gigs that you're playing. (Although, we don't know for sure, based simply upon wattage and speaker configuration.) Fortunately, it sounds like you have guitar players who don't get too crazy with the volume levels. So, you should be fine. And if they DO play too loud for the gigs, then the optimal solution is NOT to get into a wattage arms race with your guitarists. The correct solution is to turn towards the offending guitarist and say, "Hey, dummy! Turn down your @#&%# amp!" (Remember to use your most diplomatic voice. Music is all about communicating with your fellow musicians.)
. I play mostly de-tuned (C or lower) loud heavy music, so anything under, say, 500 watts has no chance of being heard unless you drop all the low mids to low end. But then it will sound crappy anyway. Standard tuning changes the fundamental tone, so a bass doesn't need so much power. 
