Onkyo A-9555 and Benz Ace L: step-up? preamp?

I have a modest system involving a Rega P3-24, an Onkyo A-9555 integrated amp and a pair of Athena AS-B2.2 speakers. I've been using the Denon DL-160 as my cart up to this point but have recently come into possession of a Benz Ace low output. The Onkyo works fine with a HOMC like the Denon, but isn't designed for a LO, so I find myself in the market for either a step-up or a pre-preamp.
While I've read through many threads here regarding the Onkyo, I haven't found any that involve this particular problem. Although I've looked at the classified offerings here, I'm utterly out of my depth and have no idea what will work with my system. Is it wise to purchase a reasonably priced tube preamp like the Soundstage Vacuum or should I stick with a step up transformer of some kind?
since the onkyo has a pretty capable pre already, a phono pre would be logical.
I'd recommend an SUT only if you know that it will work well with that particular cart. Don't know what your budget is but the ZYX pre-pre is a very good battery operated active gain head amp which works very well with a couple of different phono stages I have tried it with.
Allow me to quote Robert Deutsch's Stereophile review of the Onkyo A-9555 from the Sept. 2007 issue:

Yes, folks, the A-9555 has a phono section. It's designed to accommodate moving-magnet rather than moving-coil cartridges, and the cartridge in my phono setup is a low-output MC, so I couldn't plug it in directly. However, I still had a Bryston TF-1 step-up transformer that I'd bought many years before, and this combination worked well—in fact, although my intention was to only briefly check the sound quality, I ended up listening to several LPs that I hadn't heard for some time. The sound was well balanced from top to bottom, with a nice "open" quality. Interested in Onkyo's claim that the A-9555's phono section has been designed to optimize high-frequency transient response, I paid particular attention to its reproduction of vocal sibilants, which tend to reveal problems in this area. I heard nothing untoward: the highs were smooth and extended, with no sibilant harshness.

If your phono source consists of a Continuum Audio Labs Caliburn turntable and Clearaudio Goldfinger cartridge, you probably won't be looking at the Onkyo A-9555—but I found the A-9555's phono section far better than is to be expected at the price.
I already had a Cambridge 640P when I bought my A-9555, so I used the 640P as my phono stage from the beginning. But a few days ago I decided to try the Onkyo's built-in phono stage, and I like it better than the 640P. Not only do I now hear more clarity, bass extension, imaging, soundstage, and inner detail, it also doesn't have the midrange glare I had before with the 640P in the signal chain. I find the Onkyo phono stage more compelling to listen through.

Based on that, I'd have to say to either be prepared to get a significantly more expensive MC-capable phono stage like the Dynavector P75 or better, or get an SUT for the Onkyo's phono stage. I'm so smitten by the Onkyo's phono circuit I'd try the SUT route first.
johnnyb53 makes a great point. i would nix the benz for a better high output cartridge, and use the onkyo. no disrespect to the ace, but there are comparable carts for less money.
Keep it simple. Use the Onkyo, and trade in the ACE low output for an ACE high output or, find a good used one. The ACE is an excellent value, just as the A-9555 and it's phono section are. You won't regret it.
Let's assume for a moment that I ditch the Benz, as some of you have suggested. I just upgraded to a Rega P5 and really want to get the most out of this table. I'm intrigued by the "synergy" idea of pairing it with an Exact 2, but I bought the Benz with the (possibly mistaken) notion that a good LOMC, properly amplified, can generally sound better than an MM, as well as reducing the intrusiveness of pops and clicks.
So, setting aside monetary considerations for a moment, would pairing the Benz (or a comparable LOMC) with a preamp like the Nova Phonomena offer anything the Exact 2 (with just the Onkyo, naturally) wouldn't?
Synergy between Rega Table, Rega arm, and Rega cartridge.....This is exactly what I was considering when I upgraded my analog front end. After a ton of internet research, discussions with fellow 'Goners, and speaking with highly regarded "experts," I purchased a new Rega P3-24 table with the RB-301 arm and the Rega Exact 2 cartridge. I am thrilled with the musicality, resolution, soundstage, PRaT, and truth of timbre of this set-up, and it all works beautifully through the built-in phono stage of my Rogue Audio Cronus integrated amp.

I am sure that there's "a lot better stuff" out there, but with today's economic climate and my desire for the best way to listen to my huge collection of jazz vinyl, I am extremely happy. I'm sure that the P5/Exact 2 combination will sound wonderful straight through your Onkyo amp.....Give it a try.

By the way, I just bought a Herbie's mat to replace the Rega felt.....The Herbie's sounded considerably better than the felt when I auditioned it at my Rega dealer's place, so I'm pretty psyched to check it out in my system. And, in a few months, when I have some more dedicated "audio $$" laying around, I want to get the Rega TT-PSU. I'll keep ya'all posted. Happy Listening.
I have a similar setup and purchased a Cambridge 640P phono preamp. Works for both MM and low output MC cartridges and is really a nice improvement over the phono stage in the Onkyo.