-18dB seems to be a good middle ground / ballpark for an overall gain reduction from power amp output level to studio level:   Consider +4dBu (studio gear level) as an ideal mic-ed signal level which is defined as 1.22V. +18db amplification would bring you to roughly 10V. A Fender Deluxe Reverb i.e. can provide up to 13.3V AC RMS to it's speaker.