ALL effects pedals and multi-effects pedals can be modified. It's just a matter of locating the RC/LC analog filter circuits and changing or "modding" them. Regarding "tonesuck", multi-effects pedals have all kinds of filters in their drive/gain circuits and these are being blamed as sucking the tone out of the sound. But these filters are there for a very good purpose and that is they remove or limit noise or unwanted signal.
On a technical note, hardware modifications exposes the entire effects system, including the main processor and controller, to instabilities and, maybe, irreversible damage. Please observe that all modifications offered by third parties involve only analog signal lines in the multi-effects system. These analog modifications may involve the input and output stages, inter-stage buffers, analog control signals and the A/D-D/A pre- and post-processing circuits. Very often, the circuits modified or bypassed are the RC/LC (resistor/inductor-capacitor) filter circuits and, in the process, signals which are supposed to be filtered are allowed to pass and a slight gain is introduced in the next stage or buffer circuits giving a semblance of improving tone. However, these original RC/LC circuits are designed to roll off high and/or low frequencies or notch a certain frequency range to prevent unwanted signals such as noise, oscillations and hum/rumble from interfering with the sound output. When you take these out or shift their cut-off or center frequencies, the succeeding stages become vulnerable to unwanted signals which could cause instabilities since the next stages are designed and in fact depend on the the assumption that the there is signal processing/filtering in the previous stages.
On the other hand, regarding the digital circuitry, it is impossible to modify digital processing algorithms of these multi-effects because they are hard-coded inside the chips. These codes reside in the static memory chips inside the processors and/or controllers which are proprietary and are well guarded corporate secrets. Designers and manufacturers are also smart enough to hide the memory chips deep inside the processors because they know that if they put their algorithms/codes in external memory chips, they are exposed and become easy to read and hack.
Having said all that, I agree completely with sin.tun.ado. Reading and understanding the user's manual is the most basic requirement in using your multi-effects pedal. If you're stuck with your MFX, you have no choice but to make the most of it and be happy. If you will look at your manual, almost all or a great majority of the effects type in there have tone-altering parameter and they come in all sorts of different names such as presence, color, range, bright, and, of course, tone. Needless to say, you can tweak these to get the tone your ears are craving for. Also, analyze how each parameter at different stages changes the sound and get a good compromise between the extremes. It's also good to read articles in guitar/music magazines and books on how to get a good sound. If reading is too cumbersome (which should not be the case) or abstract for you, listen to audio books or watch video instructional materials. There are video tutorials that specifically deal with guitar tone and there are several good titles by Eric Johnson and Michael Casswell. You can also look for materials at youtube and other file hosting sites. There are also a lot of sites that offer suggestions in getting a good sound. It's easy to study. I guarantee you that if you just give time and effort to what you are studying, you will learn. Apply what you learn and, hopefully, that will solve your problem. If it doesn't, look for other solutions until you are satisfied.
But in the end, you can't satisfy everybody and everybody is a critic. For those who can't get what they want and think/believe that their only solution is to have their MFX modified, ask the technician (1) what stage he is going to modify, (2)how is that affecting the tone, (3) what is the extent of his modification, (4) how is the mod going to affect the tone, and, importantly, (5) what are the pros and cons of the modification. If it's all pros and no cons, then it's most likely that you will be conned. It's a good idea to record a few tracks of different playing styles with different patches WITHOUT NOISE REDUCTION before and after the mod. Take note of the noise level. If you're happy with the modification, then good for you. It's easy to get caught up in the bandwagon of testimonials and not understand what is going on. It's like the now banned infant milk formulas. They tell you with a lot of endorsements that it will make babies smart without telling you that it exposes your baby to all kinds of nutritional imbalance that makes your baby prone to obesity, diabetes, high blood pressure, heart disease, organ failure, cancer, etc. and you don't see the symptoms while your baby is growing up. The worst part of it is that the relationship between the milk formula and the intellectual development of the child is negligible or non-existent.
Finally, if modifications will not help you in getting the tone you want, get a different brand or get individual pedals but that's not the topic here. I must admit that there are significant differences between different brands. If, still, that won't work for you, consult a psychiatrist or talk to your pastor, priest, imam or monk.
