Is powerfull Amps only for low sensitivity speakes?

My speakers are 94db and I have twin magtechs (2 X 900 watts).  They sing, they breath and sometimes, they even cry.

I took a look at John Atkinson’s measurements of your speakers and at the description and specs of the GS150, and as far as I can tell from those documents the speakers and amp should be a fine pairing.

I assume that your preamp can provide balanced signals to the power amp, because like many ARC balanced power amp designs I’m pretty certain the GS150 will not work properly if provided with single-ended inputs via RCA-to-XLR adapters (or, for that matter, if the preamp provides single-ended signals via XLR connectors, although that would be unusual in a high quality design).

To address your initial question more generally, IMO using an amp that is more powerful than necessary can raise at least two concerns (although neither appears to be applicable in this case):

1)There tends to be a **loose** correlation between the power ratings and the gains of various amplifiers. If the overall combination of speaker sensitivity, amplifier gain, preamplifier gain, and source output level is too high, the volume control on the preamp may have to be used at very low settings, where various undesirable effects can occur.

2)Everything else being equal, more watts = more $. So with a higher powered amplifier a greater percentage of the dollars one chooses to spend may go toward power rather than quality, compared to spending the same number of dollars on a lower powered amplifier. Assuming at least that the topologies and class of operation (A, AB, or D) are similar between the amps that are being compared.

Good luck. Regards,
-- Al

An additional point against using high powered SS amps driving extra efficient speakers is most SS amplifiers produce higher levels of undesirable (higher odd order) distortion within their first watt or two just like but not quite as much as when they are pushed near their clipping limit. The other related negative side effect of such pairing is the volume controls on most preamplifiers tend to be less linear, and more coarse in some cases, at the lower operating range and become more refined at the mid-higher range. 

See here for Audio Precision measurement graphs of a 2x350 watt pro audio amplifier: http://www.homecinema-fr.com/forum/amplificateurs-de-puissance-haute-fidelite/mesures-ampli-yamaha-p...
In short, there is no problem, and the test concludes that this amplifer is excellent for domestic high quality audio systems:
CONCLUSIONS
. La puissance annoncée est largement obtenue (370W Rms sans remontée de distorsion, pour 350W Rms annoncé)
. Bande passante extra-large (ceux qui lui reprochent de manquer d’aigu, faudra m’expliquer...)
. Distorsion infime à bas niveau, très faible jusque la limite de l’écrêtage
. Pas de distorsion de croisement, toute petite remontée de distorsion dans la zone 0.5W - 8W Rms; pas mal vu le faible courant de repos de l’étage de sortie!
. Un ampli que l’on peut utiliser en utilisation domestique comme en sono de qualité
. Quant au prix... "honteusement bas" pour un appareil de ce niveau de perfs (400 euros chez Thomann...), de puissance et de protections.

Je suis content d’avoir acheté cet ampli et son grand frère P5000S pour mon système tri-amplifié, ce P3500S va donc rejoindre mon rack pour driver les médiums CMCD-JBL. Je suis convaincu, attachant une grande importance aux résultats de mesure.
(Je suis électronicien de passion et de métier)

Petite anecdote je n’ai pas entendu le ventilo se déclencher pendant les phases ou je le faisais travailler à puissance max le temps de la FFT..
La résistance de puissance, elle commençait à sentir le chaud!

Que reste-t-il aux "classe A" ésotériques et infiniment plus chers? je ne rentrerai pas dans ce débat...
Read more at http://www.homecinema-fr.com/forum/amplificateurs-de-puissance-haute-fidelite/mesures-ampli-yamaha-p...

This excellent amplifier until recently sold for 350 euro in Europe, and currently for just over $500 in the US.