La Bella make classical strings in different tensions, and also make the best sounding Silk and Steel strings to my mind (which unfortunately don't last very long). I'm not sure, though, that high tension is what is wanted here - flamenco strings are low tension, and get that way by all the strings being wound - the trebles are wound nylon on nylon. If the wound strings sound OK, there is a suggestion that wound trebles might cure the problem. High tension gives more volume but less sustain, and vice versa.