Impedance Questions

I am currently running a McIntosh 7150 amp into Martin Logan SL2's. The amp is presently set at 8 ohms. Are there any negative effects of running the system at 4 ohms? Presently the amp seems underpowered at higher intensity levels. Thanks for any advice.
Why not ask Roger Russel at ROGERR@SUNDIAL.NET He was an engineer with Mc for 20 years and a great guy.
I have no experience with that amp, but have always found that a tube amp's 4ohm tap is better than the 8ohm tap when driving Martin Logans. Better control and more neutral.