how can I use main speakers to replace sub?

I am in the unusual situation where my main speakers (Evolution acoustics MM3, 2 15" woofers in each) are much more capable in the bass than my BC acoustique sub (12"). I am hesitating between buying a pair of Fathom F113 to replace my current sub or, as a cheapest alternative to run w/o subwoofer, and have the L/R speakers reproduce the sub signal. This option makes sense as I was planning to replace my receiver anyway...

Do you know if there is any processor that allows that?
Sorry if it is a stupid question but I couldn't find how to achieve this configuration with my current Pioneer receiver: if I select "no sub" it looks like the complete sub signal is lost.
Most decent receivers will allow for "no" sub. You set the mains to "large". The unit will reroute all bass to the R/L speakers. The problem here is that bass is very taxing on an amp, especially a receiver. The speakers may be up to the task, however, your receiver probably won't have enough power to make you happy. Bass below 80hz sucks virtually all of the juice from an amp.
That's why a smaller sub may seem bad, when in reality it is a good thing to have a separate sub with its own amp. Same old story, it's not the size of the sub....quality vs quanity.
Yeah - as Elvick rightly says this is normally no problem as almost any DSP pre-amp should be able to redirect the 0.1 channel to any of the speakers you choose.
Check with Pioneer if it matters. I am not sure choosing "no sub" actually blocks the signal to the subwoofer.

That adjustment on the receiver regulates the crossover setting to the mains. All "no sub" may actually do is allow the mains to run full range. The sub still may get the same signal as before.
Dolby allows (or even suggests) decoders discard the LFE channel when no sub-woofer is present due to the excessive headroom requirements.

You'll need a preamp or receiver which doesn't do this.
Lexicon processors will downmix LFE to the main channels at a configurable level. Through the MC-1 the peak limiter could only be applied to the sub channel (I don't know about the newer models)
Your Pioneer surely will send the LFE(on DD or DTS 5.1 sources) to the front L/R speakers if you set it to "no sub", but you will need to set the front L/R speakers to "large". As others have said, I wouldn't try to crank it very loud with a receiver.

I am running like this with my Sony DA4ES and Legacy Signature II speakers with pretty good success. I don't get any where near "reference levels", however.
I run no sub with my Rotel and they sound great (I get usable bass to 16hz)