Your amps, 600W@8 ohms, have twice the rated power necessary to run your speakers. Your amps are not being maximized to their fullest potential - they are not getting enough power.
I would like to directly challenge this statement, as I have used several extremely high power amps in my system and have never had problems with having an amp that has "too much power". The fact that the amp has twice the rated power for a speaker should never be considered a negative thing. In actuality, you want a huge reserve of power supply capacitance because any time the output circuits need to pull in voltage from the power supply in response to an input signal, the actual voltage from the power supply takes a dip. Even if you can't measure this on a scope as "power supply ripple", it will still affect your sound quality and impact.
I also have 1,000 watt Emotiva XPR-1 monoblocks that include a massively huge 240,000uf power supply with a seriously heavy transformer (2.4kVA if I remember right). With Furutech fuses loaded, these amps are amazingly detailed and nice for the price. Never had a problem with "too much power for speakers", since I'm driving B&W 805, and they are specified at 120 watts. In actuality, these speakers sounded better with a beefier larger amp.
NOTE: another disclaimer on Emotiva. While they are very amazing amps for the money (if you load them with Furutech fuses), they are not the best amps out there. There is an electrical resonance in the circuits that I can hear that makes the highs a little jangly and thin, as well as some grain in the general sound. Other amps (JC1, 7b3, etc.) will be much cleaner and more refined sounding, but also significantly more expensive.
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Finally, if you really read what the OP was doing, you would see that he is in an apartment and doesn't want massive bass. While the REL subs are awesome (I have heard one), the OP will have more than enough bass with good amps on his JBL 1400.