Ki, The older A-1s had impedances in the bass of over 50 ohms. A 500 watt transistor amp will only make about 70-80 watts into that impedance. So you were never able to tap the power of the amp with that speaker.
To really get Sound Labs to sing, you need a tube amplifier that will not loose significant power into the higher impedances. The older Sound Labs IME were easier to drive, that is they were fairly efficient, and their impedance was set high enough so that the speaker could be driven easily by a 100 watt tube amp. So, Rapid, If I were you, that is what I would be looking for at the very least.
To really get Sound Labs to sing, you need a tube amplifier that will not loose significant power into the higher impedances. The older Sound Labs IME were easier to drive, that is they were fairly efficient, and their impedance was set high enough so that the speaker could be driven easily by a 100 watt tube amp. So, Rapid, If I were you, that is what I would be looking for at the very least.