I ran the TS params for the PTT4 in unibox. They went for a small Vas at the cost of low sensitivity. Maybe they want to sell more amps
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For standard Butterworth sealed, need 1.5L box (too tiny to get decent amount of audio stuffing in there to quell the internal box noise) with 85dB 2.83V/1m, & low impedance (1W sensitivity is lowly 82 dB).
Xmax is hit ~ 60W (>40Hz, ~ 100dB 1m above Fc), and it hits its mechanical limits ~ 140W (> 40 Hz, ~ 103 dB 1m above fc). No idea if it'd be happy thermally but even a simple first order in front would let this take some power (as far as throw is concerned).
Higher Vas for more sensitivity would have been a nice trade off.
The 5 kHz peak and aggressive high frequency roll off off-axis wouldn't be fun to work with. The small box TS params don't jive with the abbreviated high end. If not for these, I'd be ordering a couple.
Let us know what you hear with the 6.5's reduced distortion.
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