Hi Kingkong,
Your hum pickup is caused by poor shielding of the cable, and as said before, the shield should be driven from another opamp. See the inamp's datasheet.
Your original lowpass filter is just two cascaded RC filters so its cutoff is very slow and droopy. With your new resistors of 2.65K and 26.5K, its response at 60Hz is -6dB, which is half the voltage. Not much difference. Its slow and droopy response even cuts 6Hz a fair amount, so your 15Hz to 30Hz ECG signal is also being cut quite a lot.
Even if you use a 2-pole Sallen and Key lowpass filter, 60Hz won't be cut very much but your ECG signal will still be cut.
The Q of your notch filter is probably much too low, so it is also cutting your ECG signal. Get rid of the notch filter, it won't reduce the harmonics of 60Hz anyway.
I suggest that you use a switched-capacitor Butterworth lowpass filter IC, I used a 4-pole National one to filter all the harmonics from my sine-wave gen, but it isn't made anymore. Maxim make some good ones, even with a built-in oscillator. With a cutoff frequency of 30Hz, a Butterworth 4-pole filter will cut 60Hz 24dB, which is 1/16th of the voltage. Harmonics will be cut much more. The 15Hz to 30Hz ECG signal will barely be affected.
Hi Optikon,
The inamp has a gain of umpteen-thousands so its CMRR is probably nearly the same, and the probes will pickup some unbalanced hum anyway. It needs good shielding and filtering.