Can you overpower a speaker

The above posters are correct that you can overpower a speaker - if you hear the voice coils bottom out, which creates a smacking sound like someone hitting a metal plate with a hammer, then you've dialed in too much power and have to reduce the volume ASAP.

In my opinion, however, you should absolutely avoid powerful amplifiers for other reasons. They achieve high power by using a lot of output transistors or output tubes, which requires the use of global negative feedback to control the circuit, feedback destroying dimensionality and imparting a lifeless quality to recordings - the ear is very sensitive to the ill effects of global feedback. In addition, all of those output devices complicate the circuit, taking away subtlety and transparency. Especially with a sensitive speaker like the Legacy, it makes no sense, none, to run a high-powered amp.

Don't worry about your inabilty to run VTL MB450's - this amp is the best example of the importance of output transformers to the performance of a tube amp - the MB vintage of VTL amps used notoriously cheap output transformers and cannot drive low impedance speakers. The Wotan or Brunhilde is like having a Corvette ZR-1 with the transmission of a '65 Beetle. They were so lousy that VTL offered better transformers as a retrofit and an option. VTL didn't enter the big leagues with regard to amps until the Siegfried.

To Elevick's suggestion, it's great if you are using speakers with high-order crossovers and it will certainly give you the impression of more horsepower and protect your primary speakers, but if you are running speakers with simple first-order crossovers, then using an outboard crossover is going to kill all of the benefits that first-order crossovers bring.

Also, getting back to power generally, you ideally want an amp with a lower continuous power rating (darTZeel, Ayre, the new Rowland 625) as I explained above, but also an amp with a very stiff power supply that can deliver huge amounts of instantaneous power. Such amps, such as the big Naim amp and the CAT JL1 monos, seemingly have the power of a 500 watt Krell, but without the drawbacks.