My guess is the bellari, but the onkyo might surprise. Listen to the pre you have for while and if you feel the sound is overly dry or something is missing, add the tubes later.
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You need to listen to decide. Fortunately, the Bellari, and several other budget phono preamps, are available from mail-order sellers that offer good return policies. Try a comparison between some of these preamps and the built-in preamp on your Onkyo. Then decide if any improvements are worth the investment. Don't forget to figure a decent pair of interconnects into the equation of an outboard phono preamp.
I'm not sure what the Bellari is selling for now, but I had one some time ago when the price was $129. At that price it was ok, but there are problems with it. It may be a good choice for you, but realize not everyone had a good experience with it. If the price is still up in the $175 range, a far better choice at that price point IMO is a Musical Fidelity XLP which can be found for $165 /-.