Amp Power: Tubes vs Solid State

Does anyone have a handle on the relative power equivalance
between tube and solid state amps...i believe that a lower powered rated tube amp is capable of driving a difficult load better vs a simliar powered solid there a rule of thumb that allows a relative comparison?
That all depends what you mean by a difficult load. If it's a higher than typical impedance load many tube amps may have advantage. If it's a lower than typical impedance load many solid state amps will have an advantage. Many lower powered tube amps will clip more gracefully than a solid state amps. Many solid state amps offer more power per dollar. IMHO it's a moot point. First find the appropriate tools for the job, the decide which tool you prefer to work with.
Generally speaking,you can double a tube amps' rated power and find it compatible to a solid state{50w tube vs. 100w ss}.As Unsound said:tubes will find the most problems with low impedance and complex crossovers-unless using high amounts of negative feedback,not a good idea.Solid-state with high impediances.I hope this helps.I'm sure there are exceptions to the rule,but this is usually in-the-ballpark.
I have been told a similar relationship that Tpsonic mentions; 2 to 1 generally speaking. I had a CJ Premier 11 a 70 wpc tube amp driving a pair of Thiel 3.6's and it did a very good job. Much better than I anticipated or would have excepted given the difficult load Thiels can represent to an amp. As I found for more info and thinking of my experience the 2 to 1 sounds reasonable.
Technically speaking a Watt is a unit of measure of power, as such a Watt is a Watt. The fact that tubes clip more gracefully is similar to a stone rolling (tube Watts) down a hill (point of overload and subsequent distortion) as oppossed to a stone falling (solid state Watts) off a cliff (point of overload and subsequent distortion). Of course you can avoid frequenting the point of overload with more Watts.
Another possible factor for the phenomenon noted above is that tubes are run at much higher voltages than transistors, and therefore don't require as whomping a power supply, in addition to the fact that they don't need to utilize massive heat-sinking, traits which may endow them with more graceful performance under duress when comparing smaller, lower-powered choices built to a moderate price point.