Is a separate phono preamp worthwhile?

Hello all. First post (cherry) for me. Longtime reader.
I have a McIntosh MA8900 integrated with a Ortofon 2M Black for vinyl playback. I'm using the MM input and I'm generally satisfied. I've heard some pressings that sound amazing , so I know it's capable.Would a purchase of a dedicated phono pre add to the performance or should it wait until a future replacement of the cartridge with a probable MC in the future.I mainly listen to prog rock, metal, some jazz , psych and of course classic rock.Thanks. Greg

Separate phono stages are so much better than the cards used in amps that you would have a hard time finding one that will not be a huge improvement. Yes I know that's an expensive Mac and yes they probably say it has a really good phono stage. The thing is a phono stage has to deal with the weakest signal in all of audio. Even your relatively high output MM is still in millivolts. Line level is in volts. So not even close. The phono stage has to equalize 20dB just for RIAA, plus another 20 dB to reach line level. Any noise messes with this big time. That integrated has high voltage and makes all kinds of noise. Also the power supply is vitally important. Separate phono stage has it's own. Not even close. 

While any $500 phono stage will be a huge improvement there are some truly magnificent phono stages like the Decware ZP3 at about $1200 that will work just fine with your MM. If you move up to MC then either use a medium output cart and be okay or add a SUT to the Decware. Either way magnificent sound for very reasonable cost. Or go to the Herron VTPH2A which is more expensive but also more flexible with greater gain and able to handle MC or MM. I have the Herron and know it is superb. Decware is lifetime warranty, and Herron customer service is second to none. Others can help you find ones for less if that's what you want. The Herron is so good I would just get it and never look back.
Wait until you go to a moving coil.  The McIntosh MM inputs are considered quite good.
Not any separate phono stage would be better but great many, that's for sure. Unless it is something like Gryphon Diablo integrated's phono stage, separate is a way to go, and even then if you want higher level. Sure, one more pair of interconnects and power cord. Yet, in your case, I would wait because you don't know which MC you might choose. Cartridge first or both at the same time. I think, phono stage should be at least about twice as expensive as cartridge, especially with LOMC. 
I used an EAR 834p with my McIntosh C42 and that was much better than the internal phono preamp. When I upgrated to the C70 I tested for a couple weeks before concluding the C70 phono was better sounding than the EAR. Whatever that anecdotal evidence is worth to you.
Separate phono stages  are a big improvement to built in phono stages for the same reason as power amp and pre amp are superior to a receiver.
There are preamps that were designed for vinyl based systems. The Audio Illusions Modulus comes to mind. Later models used two 6SN7 tubes in both the phono and line stage sections. 

I once owned one. Sold it due to building a digital based system. A superior phonostage would be quite expensive. 
Point is that need of a separate phonostage is dependent the goal in the design of the preamp. 
I wanted to update yinz on my initial thread. I purchased an  'open box' Sutherland Insight w/LPS  that was in my budget and I'm amazed at the difference already.
It sounds like I've changed my speakers or front end. It's that feeling many of you describe here. So much more pleasure with vinyl.
A visual comparison would be if you were looking through a window but with no depth of field. With the new phono stage, the image is more in focus and lifelike. Just WOW. Thanks to Mr. Sutherland.

Regards,  Greg