I have tried to make sense of this thread and it seams like there are two trains of thought here...converters and reverb.
For converters my question really is what is your realistic end goal? If you are just getting started I would actually say, be conservative.
Do you want to learn sound design? Compose? Release CD's? Put up music on youtube and soundcoud?
I ask because there are folks out there doing great stuff with very conservative set ups. For around $200 there are some great 2 in and 2 out boxes from Event, Focusrite, and others that are more than fine for what you may want to do as it sounds like right now you are working in the box entirely.
Sure, Lynx, Orion, Apogee, Burl, Lavree, Benchmark and more make some great pro converters that are stellar, but each of these multi-channel boxes will set you back $2-3k or more. So its kind of like telling a starting guitarist to go buy a high end Les Paul to learn on.
The gear should serve you, your ideas and needs, not the other way around. If you have the skill and the ears which come with time and practice you CAN make killer high end and professional recordings with cheaper gear. Good gear only makes it easier.
My 1st two albums here on Hypnos from 1999 and 2000 (I think) both were done on an ADAT XT, a Mackie 1604/later a Yamaha 01V, with Shure SM-81, CAD E100 and Audix mics and mixed down through TC M3000 and Alesis Wedge reverbs. A lot of my work prior to that done in a computer was done with a sound blaster soundcard.
My point being that I have long since moved way beyond most of that gear (still have a wedge laying around) and none of it was state of the art. But I still had to start somewhere and I am quite proud of the albums I made with all of it. In fact sometimes I miss the days when it was harder and I had to really push the gear's limits and problem solve to get my results.
So, again just start somewhere with what you can afford.
This is a huge issue of taste and I am as snobby as anyone when it comes to reverb. Up until just two-three years ago I would have told you that no plug-in reverb will ever equal high-end hardware. Well now two years later I almost never turn my hardware lexicon on and I traded my Eventide HW8000 for 2 high ticket microphones as I found I could do everything it could do in the box, especially with SoundToys (the ex-eventide program team) and PSP and others...with the one exception being pitch shifting, and that should change soon.
Reverb wise though the new Spark Verb is incredible and capable of black hole like reverb, The Lexicon PCM bundle sounds virtually identical to the hardware PCM96, Softube's TSAR is great for rooms and my absolute go to reverb for big ECM and spacemusic spaces is the Lexicon 224 available by UAD (not cheap, as you need a DSP card to run it, but its still software).
Point being there are tons of choices in the computer now that can do great things. Many others listed elsewhere here.
Pick some starting tools, push them until you know you their limits and beyond and enjoy your self.
Oh and be very wary of advice on internet forums (yes this is an odd place to say this), my point is that on an audiophile forum or on gearslutz or where ever, opinions become facts! Never treat any of it as empirical data. There is no best mic pre, or best converter or best reverb. In any situation you need to hear it, touch it and use it in your space and then you can decide if its best for you.
To bring this back to your original question...yes better converters can clean up your sound and as I said there are some great $200'sh to $400'sh boxes that might be a nice step up from your Lexicon. If you can swing $500 or so street the Apogee Duet 2 is amazing in terms of sound and function, and it can actually turn the soft synths on your iPad into a real serious sounding instrument.
Ok...I will muddy the water a bit more, and will challenge you with the thought that maybe the lack openness and mud or whatever you feel is sonically lacking in your mixes is not the converters, but could be cleaned up in better mixing techniques IE equalization, compression, and even synth programing.
I am not attacking your skill, but you have only been at this a year I think you said? Audio engineering, synth programing, performing and sound design are all skills to be learned and mastered over time and in many cases each one is a separate skill, you don't need to master all of them at once.