Whether the 450 kHz filter modification would be useless or not probablyBefore carrying out a mod you have to determine what you expect.
As the author of the DT-210L & DT-400W filter mod articles, I find the
comment that the mod is 'not worth all the effort' frankly incredible!
The mods were not offered as a panacea for turning these radios into
9/10kHz-split DXers machines. That would be impossible with the type
of filters (6kHz @6dB BW) used. Rather, the articles were offered in
the spirit of improving the usability of these radios in the domestic
context. The give-away line in my article was 'the intrinsic quality
of the audio remains.' By implication therefore, the mod filter is not
especially selective merely an improvement on the original.
The moral of the story is to make sure any ULR you intend to mod for
9/10kHz-split DXing has a 455kHz IF system and the ability to tune in
at least 1kHz steps or better. 450kHz IF radios are a waste of time
since narrow filters with a good shape factor are simply unavailable.
Whilst agreeing fully with the remark by Steve about skirt selectivity
and filter size, I have to ask:
Was the LTM450HT filter used? Nothing else will bring much improvement
except for a LTM450IT or even narrower filter, if you could find one.
The 'T' suffix in the filter model number is important since these
have superior skirt selectivity over those without the 'T' suffix.
Living here in the UK I frequently wonder what North American DXers
mean by a 'high RF environment'. Amongst others I live within 15 miles
of one 150kW & two 400kW MF transmitters & less than 1 mile of two 1kW
The DT-210L & DT-400W filter mods have transformed the reception
capabilities of these radios close to these powerful signals. Before
the mod, five (9kHz) channels either side of the frequency of one of
these strong locals were rendered useless for weak signal reception by
the original filter. After the mod I am able to hear weak signals on
the immediately adjacent channels (±9kHz) with only a small level of
adjacent channel interference.
Perhaps your RF environment(s) are not really that challenging? Maybe
that is the reason you are not reaping the reward from the mod?