i ran hogtune 5.25" 4 ohms all around on my 09 Ultra without an issue for about 9 months. yes, they will sound MUCH better behind an amplifier but they will not "toast" a radio. What they will do however is throw a code if you run the radio diagnostics because the radio is looking for very specific information when in diagnsotics mode.

 

The speakers will work just fine with the stock HK head unit but if you want the most out of them you either need a 4 ohm amp or a 4 ohm head unit like the Sony I ran.

These speakers are still available and in like new condition. Check out HW web site, they are now listed as DX 504-70 and sell for $250.

 

speakers are still available