Frequency response is different and ratings as well. Sensitivity is slightly higher on 1650.3, not sure about physical differences in construction etc.
I have the 1650.3 and have heard the 165.3 albeit with a different amp. Maybe I'm a little biased but it's a very noticeable difference to me. The 165.3 aren't a bad speaker by any definition, but the 1650.3 gets louder and to my ears are more clear and full sounding and hit harder in the lower frequencies. I could have lived with the 165.3 but would have been looking for more after awhile. I knew I was happy with the 1650.3 the instant I installed them before the amp was even tweaked in.
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2018 CVO RG lava orange, 117 Stage 3, 14" F47 and a few chrome extras. Boom crap gone.
2016 RG Special Cosmic Blue, 120ST with Dragos 2:1 and TTS tuner, FBI drop seat kit and JNR air ride. Stock audio long gone.