I have the same problem, but with my k750...
first time I had flashed it successfully with a "W800" firmware..
after that, I had tried to flash it with a "W810" firmware to test what will happen..

and here my problems had started..
so, when trying reflash it with "W800" or "K750" firmwares then trying customizing it with FSX, I always get :

14:50:09| XS++ v3.2 Dev1 (Crystal) Ready !
14:50:22|
14:50:22| Attempting to open the interface...
14:50:22|
14:50:22| TURN OFF PHONE!
14:50:22| Hold 'C' button on phone and connect phone NOW.
14:50:22| You have 30 seconds...
14:50:22|
14:50:28| Baseband ID: 8040
14:50:28| Protocol Version: 3.1
14:50:28|
14:50:28| Sending db2010_cid00_prodid_p3l.bin...
14:50:28| Applet ID: 061113 1250 ALUCXC125872_COMPACTPRODUCTION_ID_LOADER P3L
14:50:28| This is a PRODUCTION_ID loader
14:50:28| Activating GDFS...
14:50:28| Warning: IMEI name does not match GDFS name (IMEI:K750 GDFS:????)
14:50:28| ...using IMEI name
14:50:28| Restarting phone - please wait...
14:50:28|
14:50:28| Disconnect phone from USB-port and wait for
14:50:28| further instructions.
14:50:28| You have 10 seconds.
14:50:28|
14:50:38| Open the battery-cover and reinsert the battery.
14:50:38| Hold C Button on phone and connect phone again.
14:50:38| You have 30 seconds.
14:50:38|
14:50:43| Baseband ID: 8040
14:50:43| Protocol Version: 3.1
14:50:43| Phone has restarted successfully...
14:50:43|
14:50:43| Profiling SEMC phone...
14:50:43| Platform: DB2010
14:50:43| OTP CID: 36
14:50:43| EROM CID: 36
14:50:43| EROM Color: Brown
14:50:43| IMEI: 35655300xxxxxx
14:50:43| Phone ID: K750
14:50:43| Region: ›5vذA
14:50:43| CDA: ›5¢ذA ›5¢ذA
14:50:43| Firmware Version: ›ç‰
14:50:43| EROM: n/a
14:50:43| Ready for operation!
14:50:47| After making any changes with FSX
14:50:47| you must press the 'Shutdown' button
14:50:52| Bypassing RSA...
14:50:52|
14:50:52| Sending db2010_cid00_prodid_r2f.bin...
14:50:52| Applet ID: 050404 1011 LLECXC125872_COMPACTPRODUCTION_ID_LOADER R2F
14:50:52| This is a PRODUCTION_ID loader
14:50:52|
14:50:52| Sending db2010_cid00_cert_r2e.bin...
14:50:52| Applet ID: 040319 0923 LLECXC1326739_COMPACTCERTLOADER R2E
14:50:52| This is a CERTIFICATE loader
14:50:52| RSA security disabled!
14:50:52|
14:50:52| Sending db2010_cid00_hack_cs_v23.bin...
14:50:54| Applet ID: 060118 0312 EMPCXC1327364_COMPACT_SEMC_CS_LOADER
14:50:54| This is a CHIPSELECT loader
14:50:54| Activating loader...
14:50:54| Activating GDFS...


I have just a usb cable (I have not a box) and until now I never find any solution..