Something with 10's for speakers. Sounds like your guitar player thinks he's a bassist...so you might as well fill up the mids! Don't try to out LOW him...you'll just ruin speakers and fart out cabs...plus muddy up the sound. It's all about the EQ in this situation. Either that, or have a good long talk with said guitar player about the sound spectrum and who fills what space!
All of the above amp recomendations sound like good ideas, just depends on the sound you want and the cash you have!
My experiances (take with a grain of salt): The Mesa 400+ cuts through great and sounds awesome, but it's heavy (42lbs), has a kinda weird EQ feature (hard to get used to at first), and is fragile cuz of the tubes (replaced mine twice in 5 years due to bangin' around on tour). The Mid section is great in this amp. It's a kinda clean tube amp, so it doesn't have an overdrive, but turning the gain all up gets a bit of distortion, if you like. Not the Grind sound like an SVT, though. Plenty of power...that's for sure!
Sansamp RBI and QSC PLX amp: So lightweight (even in a rack case) that it roadies itself to the gig!!! RBI is simply awesome in the studio. The XLR output on the back and separate level knob for it is a soundguys bit O'heaven. Good super clean to heavy overdrive sounds. EQ section is a bit limited, but suits most needs. The presence knob (super high frequency?) is about a waste of space for a rocker/punker. I keep it all the way turned down. It just adds hiss and crazy clacky/twangy sound. And I play with my fingers! I couldn't imagine it turned up with a pick!!! Unless you are going for the DD Verdi from Overkill sound...yipes! But it's a pretty inexpensive, really well built and very useful preamp. A lot of metal guys use them too!
Ampeg SVT-CL: Hey, what can I say that hasn't been written and recorded for the past 40-50 years! It sounds great and you have probably been hearing it's tone on tons of recordings and live shows throughout your life. It's expensive. Even used are still around a grand. It's a heavy bugger (78 on my scale), but it's balanced well and not too bad to move around. It always seems to find it's way through the guitars in all the situations I've used it in. So much power and that grind tone is sweet (master volume all up and gain used to control the loudness, not really overdrive...just more a gutteral-oomph!). It just sound right, to me at least! The EQ section is lacking a bit. It has that 5 selector switch for the frequency of the Mid to boost or cut, which I would rather have a variable knob. The Ultra Low button seems to scoop mids out in exchange for a slight low boost. The ultra high switch puts on that clacky/twangy sound again...if you like that, it's cool though. Almost all settings (except crazy extremes) sound good with this amp.
Sorry for the long post...good luck with your amp hunting