Decent Power Amplifier for Rotel RA 6000 Amplifier

I'd recommend a used preamp, since high end preamps last for decade and have been excellent for a long time. Conrad Johnson, Audio Research, VAC, Zesto are brands I would look at first. Although there are lots of great brands. For $3K you can get an originally $6K preamp, putting you into a very good quality range. Most of the preamps above are tubed... the preamp is a very good place to use tubes, even if you do not in other functions. 

By bottleneck @soix  means in terms of sound quality. Ultimately you want each function Streamer, CD Transport, DAC, Preamp, and amp to perform at the same level of sound quality. 

If you are looking to increase the performance of your system, you will want to use a high quality preamp and not connect your Marantz SACD 30N to an amp. 

Given what you have now, I would use the Marantz for streaming and disk playing. Get a good preamp... well, after choosing speakers. 

Thanks for the additional information. 

The rule of thumb is to buy the speakers first. Then to get the appropriate supporting electronics. This way you can carefully choose the tonal balance, power, etc to optimize the speakers which tend to make the biggest difference. 

So, I recommend a careful long look at speakers first... to find some that are emotionally involving, then look for electronics perfect to support them. 

All components of audio... speakers, amps, preamps... DACs, and streamer have unique sound characteristics. You want to carefully put these together to get the best out of them and not have them work against each other. 

If you are looking to improve the overall sound quality of your system... one function at a time... which is typical for folks pursuing high end audio. I would upgrade the preamp portion unless you are driving some really exotic and incredibly inefficient speakers. 

Rotel is good quality, particularly the amp. If you want to enter the high end, I recommend a high quality preamp, preferably tube... which is likely to be more natural and musical. As we fanatics say, the preamp is the heart and soul of the system and is critical for a great system. You should consider spending as much as was the integrated to make sure you are "upgrading" not just going sideways. You will get much more bang for the buck buying a use high end preamp. Consider, Conrad Johnson, VAC, Audio Research, Zesto... for instance. If you want new... still consider those.