How far can room treatments solve boomy bass?

If one implements the wrong solution for an acoustic problem, then yeah, it's not going to work very well. But if I have a spike at (say) 43 Hz, and I install a tuned acoustic filter (Helmholz, etc.) designed for maximum absorption at 43 Hz, and things don't get better, then I either need

a) better placement
b) more filter

This isn't voodoo.

Nope, it should work. It's probably more of *b.
I used Varitune V-6 before and more of them, absorbed more db. Using the PARC, i just deduct more level, if needed. I also have 43Hz as a major peak

For OP, if you can measure your room with XTZ RA pro II, you can order special tuned Scopus Traps. This way (measured), you can start off buying only several Scopus to start off ridding the modes. Measuring helps you and help Gik with two things.
1) Determine at what frequency or frequencies you'll have the peaks.
2) The actual level in db the modes are.

If you do not measure, you'll have to guess and might end up absorbing less specific which in turn, will not help you ridding the actual mode at that degree you'll need.
When measured and you know the actual level in db of the mode(s). You can tell that to Gik and they will be able to tell you how many panels you'll need. Also, custom make them to absorb (better/more) at this or these frequencies.

When that is done and installed, you can easily determine if you feel more panels are needed. Maybe not!
Ceiling usually is a problem. Concrete walls and ceiling can give a harder reflection. Some usually blame cables for being bright, this can be due to the ceiling, walls. Glass (windows) are even worse.
http://www.gikacoustics.com/gik_scopus_tuned_traps.html
I use 242 in the ceiling, but if i had bought now i had bought GridFusors. Placement in ceiling you can spot doing like this. Sit in your listening position, have a friend walking at each side between speakers and where you sit. When you see left speaker at left side that is first reflection, when you'll see right speaker at left side it is second reflection. Ofcourse, you can do same at right side of listening position. In line between these sides, the ceiling reflections are located. Now you can determine where to put ceiling panels aswell as side reflection panels. For example, in my room the ceiling reflection has a huge impact.
http://www.gikacoustics.com/gik_gridfusor_acoustic_diffusors.html

Bass boom:
For instance, i have a big mode at 43Hz (around 19db!).
So for me it had been wise to try Scopus and ask Gik to specially tune a kit of absorbers as close as possible to absorb at 43Hz. This i had never known if i couldn't measure my room. I can measure in corners and listening position where modes actually develops and at what specific frequencies. I can also see where in the range i have big dips (opposite to modes/peaks).

Sorry River251, just noticed your Q above about an actual HiFi-quality parametric. What I use is a Ric Schultz-modified Behringer DEQ-2496. Have a rather minimalist 2.0 system: Onkyo carousel changer as transport, out to a Monarchy DIP Combo, out to the DEQ being used as my DAC, from there to a pair of Endler Audio shunt attenuators that are plugged directly into the inputs of a pair of Monarchy monobocks. About $2k worth of Alan Maher Design's power-conditioning stuff rounds out the electronics. This is a single-sourced, CD-only system. The stock sound of DAC section of the DEQ is unacceptable for HiFi. However, many choose to use it between a transport and DAC - that way the DAC section of the DEQ itself is bypassed altogether and all the DEQ is responsible for is signal processing in the digital domain and this does improve the sound dramatically. About the only issue to deal with at this point is the cheap, stock switching power supply. It's unreliable as well as a bit noisy. Ric's mod gave it a quality linear supply. But, I took things a bit further and let Ric replace the stock analog output stage with one of his own design and opted for an updated DAC chip as well. This allowed its use as a HiFi-quality DAC with true, differentially balanced analog outs to the monoblocks. The cost for the DEQ and the mods was around a whopping $625. If you need an analog input as well, Ric can mod the existing input circuitry into something equally HiFi-worthy for a few hundred dollars more. But, for me the sound is glorious and the flexiblity is unparalelled and all that. The DEQ itself has 10 bands/ch variable from 10 octaves wide to 1/10th octave narrow at more than 330 individual center frequencies from 20-to-20k in 1/2db steps. All that with 64 user-defined memories. The processing is 32-bit. There is no volume out control on the DEQ (except for up to -15db at the DEQ's input to accomodate higher-gained sources). The DEQ has a steeper-than-analog learning curve to it, but you'll get passed that after a while...took me almost year to begin to feel really comfortable with it, but I was a bit impatient about it, as I recall. But, I dunno how to answer better your Q about what you might want without knowing your system architecture.

Thanks everyone, I'm just reading and trying to learn. When I can afford my next step, it will be getting the equipment to analyze my room.

But Ivan, I am using a Creek CD43 MkII right now, with an optical (toss link?) out so I think I can accommodate the Ric's stuff, which would be my second next step. Can you provide contact info? I should talk with him. I'm really glad that through both analytics and room treatments there is some hope.

Martykl, where in the source-speaker chain does the PARC go, and how will it affect sound quality?

Thank you all so much, this takes things to the next level. Quality sound is worth the effort...

Jim