6 ohm power with 75 watts might be a problem
Do you mean it will affect sound quality...volume? When u say "if I find I need more power"?
You possibly will have a problem with 6 ohm speakers. First you are only starting with 75 watts per channel. Comparing 6 ohm to 8 ohm nominal impedance, remembering the formula for power ( simplifying and not including inductance or capacitance is P= I^2 * R and typically unless specifcally stated AVRs are current limited. So take maximum power for 8 ohms to find maximum current = I = sqrt ( 75/8) = 3.06 amps . Now substitute 6 ohms and you total effective maximum power drops to (3.06)^2*6 = 56.25 watts
The problem is further exacerbated by the fact that ( looking at section 4 on page 6 of your owners manual) that your preouts are very limited: subwoofer and surround l/R ,mainly for a second zone. here is you manual online:
http://www.usa.denon.com/AVR-1610-OM_E_004.pdf This means it is
not possible to add addtional power to your front channels by adding and external amp; you would have to buy a new AVR.
You need to look very carefully at the sensitivity of the speakers you select, you might be able to use 6 ohm speakers, but to do so they need to be very high sensitivity ( on the order of 100 dB) and/or you need to listen at low volume, i.e. less than 56 watts . Even low effiiciency ( 80dB) 8 ohm speakers might cause problems. Average efficiency.sensitivity (90 dB) might work okay for 8 ohm speakers and high efficiency 8 ohm speakers should have no problem.
Sorry to be the bearer of bad news. Bottom line, 6 ohm speakers might not work and/or you might need to consider upgrading your AVR in the near future.
Good Luck!
Forrest Man