A brief comparison of the Dayton RS180, Scan Speak 8545, and Vifa PL18. 

Note: The Vifa was a PE buyout 4 ohm version.

Nearfield frequency response.

Linear distortion

at 600 Hz

at 1000 Hz

at 1400 Hz

Nonlinear distortion

5 tone

Single 80 Hz tone

Three tone at 150 Hz

At 800 Hz

Conclusion

All three of these drivers measured very closely. The winner with respect to linear distortion is the Dayton. But not by much. Picking a winner in regards to nonlinear distortion is more difficult. Overall, the Scan Speak had the best performance being incrementally better in all categories except one.  It would be the clear winner, except the Dayton was markedly better at 150 Hz. The nonlinear performance of the PL18 was surprisingly good, but it did worse on the stored energy tests at 1k and 1.4k than the Dayton or the SS.

If they were all the same price, I would probably pick the SS. On the other hand, you can buy 4 of the RS180's for the price of one Scan Speak. The effective nonlinear distortion of a TMM or MTM with 7" RS180's would be notably better than a single Scan Speak TM.