Will I benifit form a external phono stage

Yes, Zaikesman, you are correct, finding good low noise and low microphony tubes for something as early in the signal chain as a phono preamp is somewhat of a pain. I like to use NOS tubes which is even harder and more expensive than the new-made types. Overall, though I feel it is worth it.
I also agree with others above that upgrading the TT would be of great benefit, more so than the phono stage. I didn't mean to ignore that point, but concentrated on what seemed to be his main point of interest.

I agree with Twl that being able to optimize the cart loading will be of huge benefit. I recently got a Camelot Lancelot phonostage to replace the tubed internal one in my C-J PV-8 preamp. I am using the B-M Glider MO v.2, and found that it performed best at around 400 ohms (also, it benefitted from the use of a premium Vishay resistor - evidence that parts quality here is very important). When I compared the Lancelot at the same 47k as is standard in the C-J, there really wasn't a huge difference between the two, except for the absence of tube noise - which is where I now part company with Twl. I love tubes, but got fed up with trying to maintain the phono section and preamp with problem-free small signal tubes in the high-gain stages. Even with the best tubes I ever found, the SS Lancelot is much quieter, and tubes eventually wear out and you have to start hunting again. I'd rather be listening to music, so I'm auditioning an Innersound SS preamp as well, and could be relagating the tubes in my system to the amp alone (where I'm convinced, rightly or wrongly, that they make the biggest difference anyway).

I love my vinyl and like you have a Mcintosh C42 preamp. I own an outboard Plinius Phono preamp (great piece) (3494) and for the last two tears have used it with a Sumiko Celebration cartridge (1500). My friend has a C42 also and was using a Clearaudio Virtuoso II mm cartridge with the built in phono amp. It sounds really good. In fact, I bought a Virtuoso cartridge and am now selling my phono preamp. I have since bought a C2200 preamp with the bulit in tube phono stage and am really pleased with that. I kept the C42 and use it in another system . I have two setups using the Virtuoso and couldn't be happier. It was 599 at the Elusive Disc with any cartridge trade in. Just wanted to give you another thought - everyone seems to have good suggestions also except parting with the C42 - unless it is to get a C2200. Have fun!

yep, ph3 and p5 will reveal a bunch of rega's imperfections but you can keep rega arm if you're patient enough to adjust non-adjustable arms. also these stages "feel" what kind of cleaning did you use on your records before placing them under the needle.
ph3 is a-bit complicated on loading which is also a critical point with mc where you have to solder and un-solder different resistors to experiment. i've arranged for mine an external selector box that can be used outside of the ph3. i was intended to use this box just to measure but than solder directly the neccessary load resistor but now i simply connect this box to the arm cable(directly) and than to the phono with rca interconnects.

I agree with all the above comments. Being a former rega 25/600 owner and a former BAT P5 owner, that's a great combo, however, your TT is quickly going to be the weak link. In fact, and this is debatable I'm sure, but you may be better off with a better table/arm/cart now, then step up on the phono stage. Maybe keep the arm and go with a Aries, or Gyrodec? Or if you want to keep the rega, tweek it with a new counterweight, platter mat, and speed control.

I loved the heavyweight from Express Machining, the ringmat, and I have heard on Audio Assylum that the VPI motor controller is a huge upgrade.

sometimes there are built-in phonos that are so perfect but too rare. separate box is more sonically beneficial. ph3se is ideal with medium output carts. bat p5 can go as low as .3mV
both of them have excellent performance.

I believe you would do better to improve your TT and arm.

IMO, you will get an improvement. The two phono stages that you mention are very good ones for the money. Nothing really wrong with your HO Benz, but, in general, LO will be a little more detailed due to lighter moving parts. Being able to "load" the cartridge properly with your new phono stage will help also. Regarding the mixing of tube gear and SS gear, many people do that to get some of what they like about tube sound combined with what they like about SS. Particularly in the phono stage, since that is very early in the signal chain, the tubes will provide a very nice smooth signal for the SS stuff to work with. I expect that you will like it. You might like it enough to change the rest of your gear to tubes.

In two words or less- probably yes. Conventional wisdom is that, all things being equal, a lower output MC will outperform a higher output MC with a small part of the improvement offset by the higher gain (higher noise)needed.

With respect to phono pre-amps, you will probably hear a significant improvement with the ones you mention over the inboard unnit on the Mac. However, I would recommend that you look into an entirely different pre-amp such as the Audible Illusions with the Gold Card phono section. Superb tubed pre with superb tubed phono section. About $1500 for a good used one. There are several other great pre's with excellent phono sections out there, too. You'd improve your ENTIRE system going that route, rather than just part of the analog portion.

Regards
Jim

The PH3 and Bat are much better and more musical than the stock Mac Phono stage. You should hear better defination and more air around the instruments along with more natural timbers etc.