"As suggested, I connected my Oppo directly to the amp and found that it did make a noticeable difference. The treble was not as bad although still a tad bright. Unfortunately, I also seemed to lose a bit on the bass end. I will have to experiment so more."
It sounds like you are almost where you need to be. Try to get the system sounding as good as you can before you buy anything. Good preamps are expensive.
Start by moving your speakers a little closer to the rear wall. That should reinforce the bass. To fix the highs, try less toe in. Also, if you are listening to the speakers without the grills, put them on. If you still need more, tilt the speakers back a little. The easiest way to experiment with this is to put pennies under the front spikes on your speakers. Just go one penny at a time. If it works, remove the pennies and unscrew the front 2 spikes on each speaker until the same amount of tilt back you got with the pennies.
Keep in mind, you're only going to be able to do so much. You still have a digital source, going into a decent, but not great amp, and then to a pair of aluminium tweeters. For a high frequency problem like this, I don't have a lot of faith in fixing this by tuning the room. Other types of problems, yes. But the highs are very directional, and the sound goes from your tweeter, directly to you ear. If you can play around with it, without investing any money, by all means try it. But if the problem persists, you're going to have to change something in your system. There's no substitute for a good preamp. That said, they're expensive, and you would only be using it to achieve a band aid like effect. A preamp can't turn an aluminium tweeter into a soft dome. And for that reason, if I had to fix your problem, it would make more sense to replace the speakers. Once you start buying components in an attempt to fix other components, you have big problems when you go to upgrade. You would then need to buy components to compensate for the components that you used to compensate for the original component, that you should have gotten rid of in the first place. You don't want to go there.
