Even if using the existing 20/17 LPF "as is" on 15m, the 15m signals are ~0.5dB down and the 23.xx MHz spur is 6-7dB down vs 0 dB down on the 15/12/10 LPF. You can sharpen up the original LPF and get that 23.xx Mhz spur down to 15dB down or anything in between.
